Basic Game Mechanics

27may22: This page finalized and active; no changes are expected.

Old School Spirit
''The more rules you add the more likely it is you will constrain the players and the game. Rules are good, too many rules are bad''. -- Dave Arneson The secret we should never let the game masters know is that they don’t need any rules. -- E. Gary Gygax

Patterns, Not Rules
To the GM: These are not rules, they are patterns to help you make consistent rulings. Your will find it convenient to apply them in common situations, but feel free to change them on the fly if that better represents the situation faced by the characters in your game. These patterns are not intended to cover all possible eventualities – you will have to use use your judgement to resolve unexpected situations. Your responsibility is to make fair (and fun) rulings, not to slavishly apply a written code.

To the Players: These patterns are a resource for the GM; you should use observation and description as your tools and resources. Your knowledge of these patterns is meant to simulate knowledge which your character gained thru experimentation, practice, and experience, but which would not be fun to play out in-game. You should express any concerns in terms of that experience, not in terms of these patterns. (Eg. "When I was learning to cast this spell it always did ...," rather than, "It's written here that this spell should ....") Stay in character as much as possible.

Role-Play, Not Roll-Play
Isekai is a game of skill in a few areas where many games just rely on die rolls. Your players don’t have a “bluff” check to let then automatically fool a suspicious city guardsman, and they don’t have a “sense motive” check to tell them when someone’s lying. They have to tell you whatever tall tale you’re trying to get the city guardsman to believe. They have to decide for themselves if some NPC lying to their character.

No Limitations
In Isekai a player can describe and attempt virtually anything he can think of. He doesn’t need to have any sort of game-defined ability to do it. He can try to slide on the ground between opponents, swing from a chandelier and chop at a distant foe, taunt an opponent into running over a pit trap ... whatever he wants to try. That doesn’t, of course, mean that he’ll succeed. It’s your job to handle these attempts colorfully and fairly, choosing whatever probability you think is right and rolling some dice.

Player Expectations
Players have a right to expect that the game-world will be more or less consistent in its response to their characters, and players should be encouraged to speak up if they observe an inconsistency in game-world phenomena (no GM is perfect). However, players should be basing such concerns on their characters' experiences, not on out-of-character knowledge of so-called "rules".

Don't Roll Dice if the Situation can be Resolved through Role-Playing
Physical actions will often require a roll, but most social interactions can be resolved with little or no die rolling.  Note: If a die roll is needed, assign dice to the PC based on his roleplaying, and to the NPC based on your knowledge of his character.

Don't Roll Unless Failure is Interesting
If failure would be boring, don't call for a die roll. Just because a pattern exists for a roll doesn't mean you should use it.  Note: For example, you might instead assume success, and roll only to see how long that success takes.

If You Really Have To Roll
Each side rolls a pool of ten-sided dice. The pool with the highest single die wins. Compare the rolls to determine the winner's number of Victories, which equal the greater of: In case of a tie, remove both tied dice and consider the remaining dice; repeat as necessary. If ignoring ties leaves the loser with no dice, the winner gets only 1 Victory.
 * The winner’s highest die minus the loser’s highest die, or
 * The number of the winner’s dice which are higher than the loser’s highest die (to a maximum of 9).

Helper Patterns
The simple mechanic above is adaptable to all situations, but can sometimes require more time than is fun. The additional mechanics below include shortcuts to make play faster.

Action-Rolls, Defense-Rolls, and Second-Rolls

 * Action Rolls are for making an attack or taking any other action.
 * Defense Rolls are for resisting an attack or other action.
 * Second Rolls add dice to a die pool. A Second Roll cannot change who wins, but it can change the number of Victories. Ignore any Second Roll which reduces the rolling character’s number of Victories.

Sometimes rolls are also categorized as Aggressive or Reactive. Usually Action Rolls are Aggressive and Defense Rolls are Reactive, but Perception is always Aggressive and Stealth is always Reactive. Other exceptions exist.

Second-Rolls allow you to modify results (eg. damage) without changing probability of success. Aggressive and Reactive are used in some spell effects.

Combat Rounds
The combat round is primarily a unit of action, rather than a unit of time.

Conceptually all character actions begin together at the start of the round, and an individual character's action reaches completion when it is that character's turn to act. Thus character actions are regarded as simultaneous rather than sequential; by the time the fastest character completes his action, all the other characters are well on the way to completing their actions.

Mechanically, in each round simultaneous Action Rolls are compared to determine initiative. Then, in order of initiative, each Action Roll is compared to the target’s Defense Roll to determine success and Victories.


 * 1) Check NPC Morale
 * 2) Each character states her intended action and target, and calculates how many d10 to roll. This action and target cannot be changed except as noted in Combat Options and Maneuvers.
 * 3) All PCs and NPCs make Action Rolls simultaneously.
 * 4) All Stun Damage is removed from all characters. (This reduces the Damage Penalty to Defense Rolls.)
 * 5) The character with the highest Action Roll result performs her action.
 * 6) The target of that action makes a Defense Roll. If the target has not already acted in this round, taking damage causes his action to automatically fail (but see Heroic Action).
 * 7) Both characters make any Second Rolls needed to determine Victories.
 * 8) Repeat steps 5, 6 and 7 for each character in descending order of Action Rolls.
 * 9) “Free Speech” phase: Insults, surrenders, attempts to negotiate, cries for help, etc.

Thresholds
In some cases Bob's Action Roll must exceed a certain Threshold for his action to succeed. If the Threshold is not met the action fails and there is no need for a Defense Roll. Threshold numbers not on this chart should be rounded to the nearest value which is on the chart. In the rare case where the number of victories matters and the die roll ties with all the values listed below, consider the next value to be a 1.

Thresholds can be used instead of die rolls to save time, and can represent additional barriers to success such as range or local magic resistance. They are also used for obstacles of fixed difficulty, such as climbing a wall, and for "passive" perception, ie. being noticed by someone who is not actively looking.

Advantage and Disadvantage
With Advantage Bob counts his highest die twice; with Disadvantage he ignores his highest die. Advantage is not stackable.
 * Initiative Advantage doubles the high die only for the purpose of determining who goes first.
 * Hit Advantage doubles the high die only for the purpose of determining who wins the contest.
 * Damage Advantage doubles the high die only for the purpose of determining how many Victories the winner achieves. Damage Disadvantage can never reduce a successful hit to less than 1 Victory.

Advantage allows modification of an already-rolled result, and modification of some but not all uses of a roll. It is also a handy modifier for ad-hoc rulings.

Take-Away Dice
Some actions can act on several targets, or in several areas, or over several rounds. A roll is made normally for the first target, round, or area. Subsequent targets/rounds/areas use the same roll with the highest die removed after each target or round. Effect ends when highest remaining die < 9. Take-away area effects normally use a 15’ diameter center surrounded by 5’ wide rings.

Take-Away area: 1 round, full strength at center, -1 die for each ring. Expanding area: Multiple rounds, -1 die each round, first round affects center only, add 1 ring each round. Take-Away Targets: Full effect on primary target, -1 die on closest to target, -2 on next closest, etc.

Take-away dice allow area effects to diminish away from the center of the effect. This allows area effects to be large without being either over-powered in total or under-powered at the center of the effect.

Slow Rolls
When a random duration needs to be rolled, each roll should be delayed until needed. For example, if some effect is to last for 3d6 rounds then you roll 1d6. Roll the second die when the round count reaches the first dies result. Roll the third die when the round count reaches the sum of the first 2 dice. This simulates the character's uncertainty about the length of the effect.

Magic Numbers
Use Magic Numbers to simulate unpredictability. The Magic Number is a second random value derived from the same die roll by grouping the dice by the face (eg. 8) that they are showing and determining whether the count of dice in each group is even or odd. If the 10s group is even, the Magic Number is the count of consecutive even groups beginning with 10. If the 10s group is odd, the Magic Number is the negative of the count of consecutive odd groups beginning with 10. (Eg. a roll of 10 10 9 9 9 9 7 7 7 6 ... results in a Magic Number of +3 because 10s, 9s and 8s are even, but 7s are not.)

Most of the time a Magic Number only matters if it is less than -3 or greater than +3, but the Magic Number is always available for use if you need a quick random value. You can always generate a d4 value, and if there are enough groups you can generate a d8 value by looking at the two groups of dice which come after the Magic Number: if they match add 4 to the d4 result, otherwise use the d4 result as is.