Acrobatics: This skill makes the character more adept at grappling and avoiding diving blows, as well as recovering from falls. An Average roll will allow the character a one-step favorable shift to Unarmed Martial Arts when attempting to grapple an opponent, while a Difficult roll will allow the character a one-step favorable modification when to escape an enemy's grapple or avoid a diving blow. An Impossible roll allows a two-step favorable modification. A successful Formidable roll allows a one-step favorable modification in general Armed or Unarmed Martial Arts rolls.
Other possible uses include as a modifier to Dance; to avoid injury from a bad jump using the Jumping proficiency (allow a Difficult: Acrobatics roll to avoid injury), or to avoid injury during a botched PLF (Parachute Landing Fall) (allow a Formidable: Acrobatics roll to avoid injury under such circumstances). Acrobatics is an Agility-based skill.
Act/Bluff: Closely allied with Disguise, Act/Bluff is the ability to convincingly pretend you are something you are not. For actors, it means portraying a fictitious character, of course, but it also covers such things as effectively pretending you are holding a winning hand at poker or convincing voters that you will fulfill your campaign promises. This skill is also useful for making your way through checkpoints, spying, etc. Difficulty roles will depend on the complexity of the role and the other skills the character possesses, such as Language, Disguise, Leadership, Persuasion, Streetwise, etc. Act/Bluff is a Charisma-based skill.
Admin/Legal: The character with this skill has experience with the workings of government, bureaucracy, and the legal system. An Average skill roll will allow the character to discern the basic laws and procedures of everyday life in a community after two hours interacting with the populace. Most paper work can be successfully done with an Easy roll, more complex paperwork, such as tax returns, may take anything from Easy (1040EZ) to Formidable (tax paperwork for a major corporation). A Difficult roll will allow the character to step into a new settlement and avoid police harassment (this may be increased in difficulty for towns with draconian or arbitrary laws, and may be decreased as the character gains familiarity with the settlement).
In places where anyone cares about rank, the character may increase his chances of being promoted at the appropriate time by one step with a Difficult roll, or two steps with an Impossible roll.
A character who also has the Forgery skill may increase the effectiveness of his forged papers with this skill: one level with an Average roll, two levels with a Formidable roll, and three levels with an Impossible roll or Outstanding Success.
Catastrophic Failure with this skill could lead to anything from botched paperwork to arrest and imprisonment (or worse).
The controlling attribute for Admin/Legal is Education.
Animal Training and Handling: The character may train one type of animal per three levels of the skill possessed (skill level + Education score). Skills are Easy to teach to animals if four weeks are spent on each trick, Average if three weeks are spent, Difficult if two weeks are spent, Formidable if two weeks are spent, and Impossible if one week is spent. At the GM's option, the difficulty can be modified for the animal's intelligence is higher or lower than average. Most wild animals can't be trained without considerable difficulty and reinforcement; wild animals should take four times as long to train. Outstanding Success allows the handler to train the animal in half the time, or teach one additional trick in the normal amount of time. Normal failure means the animal does not grasp the concept of the task; the handler may try another task roll at one difficulty level better in another week. Catastrophic Failure means the handler will never be able to teach the animal that particular trick; however, another character with this skill may try to teach the animal the same trick.
The controlling attribute for Animal Training and Handling is Education.
Archaic Artillery: This is treated just as Heavy Artillery skill, but it is used only for the old style black powder and cannonball artillery popular in the 19th century. Think American Civil War.
Bribery: The character with this skill has the experience with bribing petty and not-so-petty officials in order to circumvent regulations and laws. Minor officials may generally be bribed to ignore local laws and regulations on an Easy to Difficult roll, depending on the bribe given. If the first offer is refused, a second offer may be made, with double the value of goods or services, at the same difficulty level. More important officials will require more difficult rolls and higher amounts of bribes. What bribes will cause a positive reaction is highly subjective and varies with the official being bribed.
Other skills may being modifiers to the difficulty level, most notably Persuasion and Carousing. Bribery is a Charisma-based skill.
Carousing: This is the skill that social butterflies have. They are able to make friends and acquaintances easily, skillfully join conversations and make witty remarks, and generally blend into any social situations. A successful Easy: Carousing roll can turn persons encountered in a "Friendly and Curious" manner (see p. 167 of the Twilight: 2000 Version 2.2 rules) into trusted allies who will do favors. Average: Carousing will turn an "Open to Contact" encounter into "Friendly and Curious;" Difficult: Carousing will turn "Neutral" into "Open to Contact," a Formidable check will turn "Suspicious" into "Neutral,", etc. Characters with this skill may attempt greater encounter shifts; i.e., the character may attempt to turn "Neutral" into "Friendly and Curious" with a Formidable roll. In order to use this skill, the character must interact, and preferably, converse in the same language as the persons encountered. Adjustments may be made for bribery, but threats don't work in conjunction with this skill; in fact, they hinder it. If the character can speak only a language in the same group, increase the difficulty by one step; if they cannot speak the language or a related language at all, increase the difficulty by two steps. Carousing is a Charisma-based skill.
Carpenter: Basic items (simple furniture, simple woodwork, basic housing) may be done without plans by a carpenter with an Average: Carpentry check. Fancy furniture requires plans (Average: Carpentry) or may be done without plans (Difficult: Carpentry). More complicated items absolutely require plans and should be graded by the GM on difficulty; something like a catapult might require a Formidable: Carpentry check, while a crossbow would require a Difficult: Carpentry check. For most carpentry activities, at least basic tools are required; for some tasks, power tools may be required or speed the job considerably. The controlling attribute for Carpentry is Strength.
Cartography: While anyone may draw a simple route sketch, the character with Cartography skill may make detailed maps that can be followed by anyone and used to plan timetables and in precise land navigation. Most simple maps (such as a street with addresses and accurate sizing of houses) are Easy. An entire village may be drawn with an Average roll, towns may be drawn with a Difficult roll, cities with a Formidable roll, and major cities with an Impossible roll. (Alternatively, the task of large areas may be broken into smaller tasks, each with a lesser difficulty roll). Open areas may be drawn in a similar fashion, but the difficulty of the area to be mapped depends upon the size and the amount of landmarks. Obviously, a trackless desert or dense jungle will be Impossible (or even worse), while an open area with numerous hills, valleys, buildings, roads and glens will be much easier (perhaps Average-Easy). Remember, in warfare, accurate intelligence data is life.
The controlling attribute for Cartography is Education.
Cryptography: The character with this ability has the ability to devise and break codes. Simple codes can be made on an Easy: Cryptography roll, and broken on an Average roll; something like a CEOI allows a Difficult: Cryptography roll, with a Formidable toll to break. Other codes are correspondingly more difficult to make and break. Radio scrambler codes typically take a computer and a Formidable roll to break, while the hopset of a frequency-hopping radio requires at least an Impossible roll, is not worse. Cryptography is an Intelligence-based roll.
Dance: Dance has very few game applications by itself, though it may modify a variety of skills, including Carousing, Streetwise, Disguise, and possibly others. Two possible skill modifications are Unarmed Martial Arts and Armed Martial Arts; for every 5 levels of Dance skill, allow a DM of +1 per 5 points of the Dance skill score (modified by Agility, rounding down) to both of these skills. Dance is an Agility-based skill.
Escape Skill: This skill's controlling attribute is Agility. It allows a character to slip out of bonds and bindings. Some difficulty ratings of typical bindings are: string (Easy), cord or rope (Average), handcuffs, rawhide, or plastic quick-ties (Difficult), shackles (Formidable), and straitjackets (Impossible). Success with other types of binding materials may be extrapolated from this information. When using a combination of binding materials, add together the difficulty levels; thus, if the character is bound with handcuffs and a straitjacket, the difficulty should be something like Extremely Impossible (one-sixteenth normal chances). Escape skill does not grant the ability to pick locks, even on handcuffs and shackles; for that, you need Intrusion skill. Escape skill merely allows one to slip out of bonds by manipulation of joints and muscles, and, to a certain extent by loosening bonds such as knots.
Time required to execute this skill is typically 3d6 phases per difficulty level. Chance for success may be increased by one level per extra 3d6 phases spent preparing for the escape attempt.
Genetics: Used in conjunction with farming, this skill allows a one step favorable shift in the difficulty level of farming for every growing season of study. Used in conjunction with raising animals, it allows the same shift for items such as meat production on farmed animals or milk production. This skill may also be used for breeding purposes to help eliminate genetic diseases or to breed animals with certain desired characteristics, such as breeds of dogs. A scarier aspect of this skill would be eugenics, breeding humans with certain desired characteristics. Use of this skill takes years sometimes, and a lot of patience. Genetics is an Education-based skill.
Ground Vehicle (Snowmobile): As it sounds, this is the skill required to operate a snowmobile. It is not the skill required to operate over-the-snow vehicles such as the BV-206 (NATO Combat Vehicles Guide); the skill required for vehicles like that is Ground Vehicle (Tracked). Ground Vehicle (Snowmobile) is otherwise treated the same as other Ground Vehicle skills.
Jeweler: A character with skill may make jewelry and cut gemstones. Simple lapidary and such requires an Easy: Jeweler check. Worked metal objects require a Difficult: Jeweler check. Engraving may be done with appropriate tools and the check is Average-Formidable (depending on the detail required). Gem cutting is Formidable: Jeweler, and requires special tools; it cannot be done without these tools. The character may increase the value of metal or gemstones by 10% for every 1 point by which the character beats the required skill roll. Outstanding success adds another 100% and creates a work of art, or halves the required time for the job. The controlling attribute for Jeweler skill is Agility.
Jumping: The controlling attribute for Jumping is Strength. The following can be used as general guidelines for the use of this skill: jump upwards from a standing start 75% of your height (Easy), jump upwards from a standing start your height (Average), jump upwards from a standing start 125% of your own height (Difficult), jump upwards from a standing start 135% your own height (Formidable), jump upwards from a standing start 150% your own height (Impossible). Add one level of difficulty if you're carrying 25% of your maximum load, add two levels if you're carrying 50% of your maximum load, add three levels if you're carrying 75% of your maximum load, and add four levels if you're carrying 100% of your maximum load. Outstanding Success indicates a good landing (such as on your feet), or an extra 50% in height. Catastrophic Failure means that you fall on your butt.
Also from a standing start: long jump twice your height (Easy), long jump 225% your height (Average), long jump 250% your height (Difficult), long jump 275% your height (Formidable), long jump 300% your height (Impossible). Modify for weight carried as per high jumps above. Landings are usually on the feet, but outstanding success means an extra 50% of the distance jumped. Catastrophic Failure could be truly catastrophic, if you were trying to jump a gap between buildings, for example.
From a running start: long jump 300% your own height (Easy), long jump 350% your own height (Average), long jump 400% your own height (Difficult), long jump 450% your own height (Formidable), long jump 500% your own height (Impossible). Add difficulty levels as above. Once again, Catastrophic Failure could be truly catastrophic, but Outstanding Success indicates an excellent landing or an extra 50% of distance jumped.
Standing or running: jump downward twice your own height without hurting yourself (Easy), jump downward 300% of your height without hurting yourself (Average), jump downward 400% your own height without hurting yourself (Difficult), jump downward 500% your own height without hurting yourself (Formidable), jump downward 600% your own height without hurting yourself (Impossible). Modify for weight carried as above. Landings are normally in a tuck and roll, but Outstanding Success allows for a standing-up landing. Catastrophic Failure causes 1d6 points of damage per difficulty level of the jump to each leg and 1d2 damage per difficulty level applied to one other random body part. Normal failure causes 1d6-2 damage per difficulty level to each leg.
Jumps can be combined, at a total penalty of one difficulty level to the jump.
Long Distance Marching: This is the skill of forced marching. A character with this skill may march for one extra period with an Easy: Long Distance Marching roll, two extra periods with an Average roll, and three extra periods with a Difficult roll. The character may also march the normal distance per day without fatigue on a Difficult roll. A character with this skill may carry his full weight allowance without a speed penalty on a Difficult roll. A character with this skill may attempt to march faster with this skill; +15% distance is done with an Easy roll, +25% distance is done with an Average roll, +35% distance is done with a Difficult roll, +50% distance is done with a Formidable roll, and +75% distance is done with an Impossible roll. Finally, the character may attempt to march with a greater than normal load, but the normal distance; +20% is an Easy roll, +40% is an Average roll, +60% is a Difficult roll, +80% is a Formidable roll, and +100% is an Impossible roll. Combinations of these are possible by adding together the difficulty rolls; i.e.; marching +15% distance with +20% load for one extra period would be a Difficult: Long Distance Marching task. The controlling Attribute for Long Distance Marching is Constitution.
Mason: With the appropriate stone working tools, the character with this skill may build items and structures out of stone, bricks, or concrete. Sculpture is also possible; this requires a Formidable: Mason roll. Brick or cinder block structures may be built using an Average: Mason roll. Structures may also be built from unworked stone; this usually requires a Difficult: Mason roll, but simple structures such as a low wall for cover will require only an Average: Mason roll. Concrete walks, floors, and driveways may be poured using the appropriate equipment and an Average: Mason roll. Repairs may be made to existing structures; Easy for 1-10% damage, Average for 11-20% damage, Difficult for 21-30% damage, Formidable for 31-40% damage, and Impossible for 41-50% damage. (Structures more damaged than this must be razed and rebuilt from scratch.) Even a dam may be built, using a combination of this skill and Civil Engineer; this should be a Formidable: Mason roll under most circumstances. Note that in most cases, the Mason will not be doing all the actual work himself; he will be supervising other (not necessarily Mason-skilled) individuals. Mason is a Strength-based skill.
Mortar: This skill is generally used in the same manner as Heavy Artillery, but is used with Mortars. (Mortar gunnery and grenade launcher gunnery are sufficiently different that the skills cannot generally be used interchangeably.) Most mortar gunnery is Formidable: Mortar on the first shot, and this can be improved with corrections in the same manner as artillery fire. Direct lay (where the gunner can see the target) is Difficult. Blind fire (where the crew knows the location of the enemy on the map, but cannot see the target and has no forward observer) is Impossible, and in any case, the crew will not know the results of their fire and must guess when to stop. Fire against a pre-surveyed point (a point on the map well known, marked, and surveyed for accuracy, with pre-determined coordinates) is one level easier. If the mortar is capable of direct fire (such as some French or Russian mortars), the fire is resolved using Grenade Launcher skill.
Music: This skill is the ability to play some sort of musical instrument. Aside from entertainment value, this skill can smooth over encounters and perhaps provide the character some employment. At the GM's discretion, it may modify Act/Bluff, Carousing, Disguise, Streetwise, Persuasion, Willpower, or Recruiting. Music is an Agility-based skill.
NBC Specialist: This skill brings knowledge of how to survive on a contaminated battlefield. The character with this skill has knowledge of decontamination procedures, countermeasures against chemical, biological, and nuclear exposure, and how to properly read Geiger counters and chemical sniffers. This skill is required for proper operation of the Steam Decontamination Trailer and the NBC Decontamination Trailer. A character with the NBC Specialist skill can also maintain equipment used for water decontamination and desalination. He may also maintain NBC protection gear such as protective masks and clothing, and determine whether they are still safe to use. He may determine whether or not a given area is safe to camp in an NBC context. Finally, much like the Biology skill, the character with this skill may determine whether plants, animals, and foodstuffs are safe to eat and drink after examining them. NBC Specialist is an Education-based skill.
Painting: This is another skill that may provide the character with employment or an item of some salable value. People that can paint can usually draw, and this may aid in some encounters. A very possible game use is to compose effective camouflage patterns on equipment and vehicles; a vehicle, building, or piece of equipment worked on by a painter with access to the proper paint colors is one level more difficult to spot on a Difficult roll, two levels more difficult to spot on a Formidable roll, and three levels more difficult on an Impossible or Outstanding Success roll. (Remember that camouflage patterns must change with the terrain, and eventually wear off and fade.) Catastrophic Failure at the camouflage task makes the vehicle or equipment one level easier to spot, while Normal Failure doesn't help or hinder the vehicle or equipment. Most difficulty rolls with this skill will be Average or Difficult. Painting is an Agility-based skill.
Physics: The primary uses of Physics in game terms are to determine how far radioactive material is away using readings from a Geiger counter, and to determine the degree of danger in the present and the future from radioactive materials. Most determinations will be Average tasks. A character with Physics and a successful roll will be able to take a reading and determine how many rads a character will take in a given area, before exposure. Another possible use is to examine a nuclear bomb and to determine the yield of the device. Physics is an Education-based skill.
Pickpocket: This is the ability to take something off someone else's person without the victim's noticing that the object is being removed. Most items (stereotypically coins and wallets) may be removed by an Average opposed roll of the characters Pickpocket skill and the victim's Observation skill. Pistols and knives may be removed from a closed holster or sheath by a Difficult roll. If the holster has a safety loop, the roll is Formidable. If the victim is distracted, decrease difficulty by one step. Items may be taken out of closed backpacks or equipment pouches with a Formidable roll. If the item is tied to the victim (such as with a "dummy cord" tied from the item to a pistol belt), the roll should be Impossible. Pickpocket is an Agility-based skill.
Pilot (Fabric Wing): This is the skill required to fly hang gliders and ultralights. It is otherwise treated as normal Pilot (Fixed Wing) skill.
Pilot (Glider): This skill is required to fly sailplanes and gliders, even heavy ones such as those used to deliver troops during World War 2. If the pilot has this skill, he may also use to attempt to safely land deadstick aircraft (those on which all engines have failed) using his Pilot (Glider) skill. It is otherwise treated as normal Pilot (Fixed Wing) skill. Hang gliders cannot be flown with this skill; that requires Pilot (Fabric Wing) skill.
Pilot (Jet): This is the skill required to fly high-performance jet aircraft, such as fighters and attack aircraft. It is otherwise treated as normal Pilot (Fixed Wing) skill.
Pilot (Multiengine): Normal sized aircraft, even those with multiple engines, are flown with the appropriate Pilot (Fixed Wing), Pilot (Jet), or Pilot (Rotary Wing). Pilot (Multiengine) is required to fly heavy, multiengine aircraft, such as airliners and cargo aircraft such as the C-130 or C-141. It is otherwise the same as normal Pilot skill.
Pilot (Spacecraft): As it sounds, this skill is the one required to fly spacecraft. This skill, Pilot (Jet), and Pilot (Multiengine) are required to fly the Space Shuttle within the atmosphere. (Average the three skills together when determining skill level for purposes of flying the Space Shuttle or Russia's Buran).
Psychology: This skill allows a character to judge the motives of another character. It can be used, for example, to determine the truth or falsehood of statements the character makes. It can also modify other skills, such as Disguise, Act/Bluff, Instruction, Interrogation, Persuasion, Streetwise, and Leadership. If the GM uses it, it may also be used to treat Post-Traumatic Stress Disorder and other mental illnesses. The controlling attribute for Psychology is Intelligence.
Reactor Operations: The character with this skill is able to run, repair, fuel, and service a nuclear reactor, or supervise others to do these tasks. Many specialized tools and materials are necessary for these tasks. Most difficulty rolls for these tasks will be Average to Formidable. The controlling attribute is Education.
Recruiting: The character with this skill is able to determine the potential of candidates for tasks, and then convince them that service under the command of the character or his associates is the best option. Recruiting novice troops takes an Easy: Recruiting roll. Recruiting Experienced troops requires an Average roll. Recruiting Veteran troops takes a Formidable roll, and recruiting Elite troops requires an Impossible roll. These difficulty levels assume recruiting from the average populace; the GM may adjust down or upward depending on what information he has about the quality of individuals the area has. The player may recruit 10% of the available labor pool for every two points by which he beats the required skill roll. (Rolling exactly what you need will recruit 10% of the labor pool in that category; at least one individual will be recruited per successful roll.) GM may adjust the difficulty of the rolls based on bribes, promises (if believed), etc. Other circumstances, such as whether the recruited individual has to leave home or not, should also be considered. Under most circumstances, the recruiter will have to be able to speak the language of the labor pool; if he cannot, adjust the difficulty by one step. Recruiting is not always for military purposes. Recruiting is a Charisma-based skill.
Research: This is the skill that scientists and intelligence analysts have. It allows the character to see patterns in things that the average person cannot, or to see how seemingly disparate facts fit together into a logical whole. When used to find out information, the GM should devise a difficulty level based on the information looked for and the size of the community where the character is looking, as well as the availability of books and other such information.
When this skill is used by to create battlefield intelligence, the GM should base the difficulty on how much information about enemy movements and capabilities is available. Success allows the character to accurately predict what the enemy is going to do within the next few hours, and the information can then be used to devise countermeasures. This skill is not the same as Tactics; success with Research requires much more time, and produces more lasting results.
The controlling attribute for Research is Education.
Sculpture: Though this skill has very little game use, it may provide the character with employment or a salable object. A possible use is whittling or carving a weapon, whether real (as in some kind of blade or arrowhead), or fake to fool someone (as John Dillinger did to break out of prison). Difficulty level depends on the complexity of the task. The controlling attribute is Strength.
Ship Helm Operations: This is the ability to run or act as a crewman of a large ship, such as a Naval Vessel. Most tasks will not require a skill roll, but operation of the ship in difficult circumstances, such as combat movement or during a storm, will require a roll at a difficulty level determined by the GM. This is an Intelligence-based skill.
Sniping: Sniping is a Strength-based skill. It manifests itself as a modifier to the base range of firearms. To determine the modifier, take the modified Sniping score (Sniping skill modified by Strength), subtract 7, and divide by 2. This is the multiplier that is used to modify the base range of the weapon.
For example: John Thomas has a sniping score of 5 and a Strength of 7. His Sniping score, modified by Strength, is 12. Subtracting 7, we get 5. Dividing by 2, we get 2.5. The base range of eligible weapons is multiplied by 2.5 to determine his new base range with eligible weapons. The minimum multiplier is x1.1. Base range is modified by the scope before multiplying.
The following weapons are eligible for Sniping score modifications: automatic pistols, revolvers, special purpose pistols, all rifles, crossbows, and automatic rifles. The Sniping range applies to a maximum or three non-automatic shots per phase. The weapon must have some sort of sight, such as laser sights or telescopic sights. One phase must be spent aiming before taking the Sniping-modified shots.
Song: Depending on skill level, the character has some sort of singing voice, whether decent or truly great. Aside from entertainment value, this skill may, at the GM's discretion, modify Act/Bluff, Carousing, Disguise, Persuasion, Recruiting, Streetwise, or Willpower. It may also provide the character with some employment. Song is a Charisma-based skill.
Stalking: Stalking is the urban equivalent of Tracking. It involves following a quarry that typically does not want to be followed. But instead of looking for prints and droppings, the stalker is skilled at judging the quarry's path by watching for unconscious signs from bystanders, gauging distances to alleys, peering into shadows, and suchlike. Characters adept at Tracking may substitute that skill for Stalking at one difficulty level higher than usual. The opposite is also true. Stalking is an Intelligence-based skill.
Streetwise: This skill simulates street smarts and some common sense. Streetwise is a general knowledge of how to survive in the roughest parts of an urban environment. It includes such things as knowing how to project an image that engenders respect among the locals, and knowing where to go to find what the character needs. Black markets may be found with an Average: Streetwise roll. Casing an urban location is a Difficult (Streetwise + Observation/2) roll. Facing down a street tough is a Difficult: Streetwise roll (modified up or down for the relative size and Strength of the two involved, and how well-armed they are). Facing down a gang is Formidable, modified as per facing down an individual, and further modified for the size of the groups involved. Obtaining false papers or otherwise finding a forger is a difficult task. The controlling attribute for Streetwise is Charisma.
Submarine Helm Operations: This is the same as Ship Helm Operations, but applies to the three-dimensional world of underwater combat.
Tactics: This is the talent of staying a step or two ahead of your enemy, and devising plans to take advantage of this. This skill deals with small unit tactics, up to about 1000 men, five or 6 warships, or about a squadron of aircraft. Because tactical skill is an intangible, the Tactics skill is left open to GM interpretation, but the skill should allow the character to plan highly-efficient ambushes, discern enemy movements and plans a few minutes ahead (or 1-6 turns ahead for Last Battle games), or help to discover hidden movement. The GM should give the player some hints on what to do in such cases, or a little help in devising defenses. Encourage the player to read books on combat tactics. Tactics is an Intelligence-based skill.
Veterinarian: This skill may basically be treated the same as Medical skill, but for animals. The veterinarian may attempt to use his skill to treat humans, but at two difficulty levels higher. Veterinarian is also an Education-based skill.
Willpower: This is the ability to keep going when all hope seems lost. A character may ignore the effects of a "Scratch" level wound on an Easy: Willpower roll. "Slight" wound effect may be treated as a "Scratch" with an Average roll. "Serious" wounds may be treated as "Slight" wounds for detrimental effects on a Formidable Roll. Critical wounds (except to the head) may be treated as "Serious" wounds for ill effects on an Impossible roll.
Another use of Willpower is to decrease the difficulty of feats or other skills that are based on Strength or Constitution. A successful Formidable: Willpower roll decreases the difficulty of feats requiring those skills by one level. An Impossible Willpower skill roll decreases the difficulty by two levels.
Willpower is an Intelligence-based skill.