The Core Rules

A game of the Infinite Horror TRPG is usually played by one Storyteller (ST from here on) and several players, whose characters are the PCs (player characters). The ST creates the world and the starting point of the story for the PCs, and answers the PCs' actions from the world's point of view. As the voice of the world, the ST should hold neither goodwill nor ill will toward anything in it. The PCs carry the story forward, and should take responsibility for their own actions.

When the rules conflict with the ST's common sense, the ST may change the rules to fit it. This is called a house rule (HR from here on). Naturally, if a house rule is unacceptable to the PCs, they have every right to flip the table.

Rolls: Static and Opposed

The Infinite Horror tabletop roleplaying game uses ten-sided dice (written d10 from here on) and counts successes. Whenever your character tries to do something, and success or failure would lead to different outcomes, make a roll for them. It usually goes like this:

Work out your dice pool. Your dice pool (DP) decides how many d10s you roll. Usually it is one Attribute + one Skill + or - other modifiers, but depending on the action's description or the ST's judgment, a DP may instead be a single Attribute, the sum of two Attributes, or a special fixed number.
Work out your again level. If any die in the roll shows a 10, you may roll one more die, and if it succeeds, add that success to your total. If that die also shows a 10, roll another, and so on. This is called the "10-again" rule. Some abilities or items grant "9-again", meaning a 9 or a 10 both let you roll another die. The best possible again level is 8. A simple comparison may help: the board game Aeroplane Chess uses a six-sided die and a "6-again" rule, so every time you roll a 6, you roll again.
Roll. Roll as many d10s as your DP. Each die showing 8, 9, or 10 is one success. The total number of dice that succeed is your successes.
Add your extra successes. High Attributes and Skills give extra successes. An Attribute of 6 gives 1 extra success, and every 5 points above that gives one more (so an Attribute of 11 gives 2). Skills follow their own rule; see the Skills Overview page. You can gain extra successes in other ways too. If a roll ends with more than 0 real successes, add your extra successes to the total. (See "Using Extra Successes" below.)
Compare with the target. The difficulty (DC from here on) is the target number for the roll. If your successes equal or exceed the DC, the action succeeds, and the more you beat it by, the better it goes. Otherwise the action fails. Some rolls care about no target at all, only about how many successes you get, which decides how well you do: how much damage an attack deals, how high a leap goes. The DC depends on whether the roll is static or opposed.

Static roll: a roll against a fixed, preset DC, such as lifting a sack of rice, jumping a pit, or climbing a wall.

Opposed roll: a roll whose DC is the number of successes an opponent rolls, such as beating someone in a race, or trying to knock someone several meters back.

For example, Player A wants to stab Player B in the back, and sneaks up on them. Neither player has any device or ability for detection. Player A rolls DEX + Stealth, and the successes become Player B's DC to notice. Only if Player B's PER + Investigation roll beats that DC does Player B spot Player A. Otherwise, Player B takes a knife without ever knowing it was coming.

Chance Die

Now and then, for one reason or another, the number of d10s you can roll drops to 0 or less. You can still roll one d10. It only counts as a success on a 10, and it uses the again rule as normal. If it shows a 1, you suffer some setback (set by the ST). This is called a dramatic failure. A roll like this is called a chance die. Although it is called a roll, it is not really one: it is unaffected by any ability that adds to a roll's DP or result, unless that ability says otherwise. And if the chance die has already succeeded, a 1 on a later again die does not count as a dramatic failure.

Using Extra Successes

Extra successes are added on top of an action and make it go better, but they can never turn a failed action into a success. In other words, if your rolled successes come to 0 or less, the action fails no matter how many extra successes you have.

For example, if a roll gets 1 success but you lose 1 success for some reason, your result drops to 0 and you cannot add extra successes.

The extra successes a character has on attacks are called attack extra successes (including those from equipment, Attributes, Skills, upgrades, and so on).

Every attack action a character takes in a round can use all of their attack extra successes.

If one attack action can make several attacks, the attacker must declare, before the attack, how the action's attack extra successes are split among those attacks.

Special: when a character attacks several targets with one attack action, such as attacking each target in an area once, each target receives all of the extra successes declared for that attack.

Special: when one attack action both attacks several targets and attacks some targets more than once, every target attacked once receives all of the attack extra successes, while the attack extra successes must still be split among the attacks on a target attacked more than once.

Special: when a character attacks one target several times in one attack action and gains attack extra successes partway through (on a hit, from a roll, and so on), the character may use them right away, but once this has triggered, it does not trigger again during the same action.

Special: if a character uses several weapons to make multiple attacks against one target, or to attack several targets, attack extra successes are split separately for each weapon used. (For example, if one attack action uses an ordinary longsword and a greataxe that gives +4 extra successes on attacks, the greataxe's 4 extra successes can only be split among the greataxe's own attacks, not moved to the longsword's.)

The extra successes a character has on defense are called defense extra successes.

To use defense extra successes, a character must declare how many they will use before the attacker rolls. If the defender's extra successes (and any ability that reduces the attacker's successes, but not abilities like damage absorption, reduction, or negation) cancel all of the successes the attacker's dice actually rolled (before the attacker's extra successes are counted), the attack fails (misses). The attacker's extra successes then have no effect, and none of the attack's on-hit effects apply.

The character then loses the declared number of defense extra successes for the rest of the turn. They come back at the character's next action.

Special: when a character's Base Defense drops to 0, for example from charging, multiple-attack penalties, or High Speed, they lose the defense extra successes from Legendary Dexterity and Legendary Perception.

Things to keep in mind:

Defense extra successes must be declared before the attack is rolled. So a defender may declare 5 extra successes while the attacker rolls only 1 success. In that case the defender's other 4 extra successes are unavoidably wasted.

A defender may also forget (or, for some other reason, fail) to declare defense extra successes before the attack is rolled. In that case, they are treated as using none.

If the ST prefers a more relaxed game, though, they may let players, when defending, assign defense extra successes freely after seeing the attacker's successes.

Rounding: Unless stated otherwise, when a calculation gives a fraction, drop everything after the decimal point. This is called rounding down.

What "/" means in the rules

"/" usually means division: the number before it divided by the number after it. For example, in "6/Resolve extra successes + 1", if your Resolve extra successes are 1, the result is 3. There is one exception: in something like "1/minute", what follows is only a unit with no number, so the result is 1 minute.

Multiple Multipliers and Additions

Sometimes the rules have you multiply a value by a number (or by several times a number), by a rolled value, or add a number to it. With only one multiplier or one addition, simply multiply or add as normal. When two or more multipliers apply to the same abstract value (such as a modifier, a rolled value, or successes), or multipliers and additions apply together, treat each multiplier as adding (that multiplier - 1) times the value, and add the additions last.

Example: a character whose Bloodline doubles their land speed uses an ability that temporarily makes their land speed four times normal, and wears a pair of boots that add 50 to land speed. Their actual land speed is increased by 1 times, then by 3 times, then by 50: land speed x (1 + 1 + 3) + 50.

Note that this rule covers calculations on one single value. For a value derived from another value, finish calculating the first value before working out the derived one, rather than mixing the two calculations.

Example: a character's base speed is doubled and increased by 10, and then their land speed becomes four times as fast. Their actual land speed is (base speed x 2 + 10) x 4.

Division:

Some rules give you abilities that halve something, like a cost or the time needed to rest. If you have several, apply all the multiplication and division first as normal, then the addition and subtraction.

Example: your Bloodline halves your mana costs, you then use an ability that halves mana costs, and you then pick up a weapon that lowers mana costs by 1. Your actual mana cost is one quarter of the base cost, minus 1.

These calculations do not apply to things other than the player's own numbers. For money, distance, and the like, use ordinary arithmetic.