Xero Sum Engine SRD | XeroSumGames

CORE MECHANICS

DICE CHECKS

Whenever a character takes an action or task that affects the narrative, a dice check (also known as a dice roll) is necessary to clearly define the outcome.

To make a dice check, roll 2 six-sided dice (written as 2d6) and add or subtract modifiers for skills, attributes, and conditions.

A total score of 9 or above is a Success, with higher and lower scores providing different levels of success and failure (see Outcomes).

TABLE 1: OUTCOMES

MODIFIERS
Dice checks can be influenced by up to three modifiers – attributes, skills, and conditional. These modifiers are cumulative and are all added or subtracted from the result of the 2d6 roll for a final total score.

ATTRIBUTE MODIFIERS (AMods)
Each character has five attributes – Reason, Acumen, Physicality, Influence, Dexterity. These range from -2 (Lame) to +4 (Human Peak). This number is the Attribute Modifier (or AMod) that is applied to any dice check that uses that attribute (when using Reason to solving a puzzle, for example).

TABLE 2: ATTRIBUTE MODIFIERS

SKILL MODIFIERS (SMods)
Each character has a variety of skills that range from -3 to +4 based on their expertise. This number is the Skill Modifier (or SMod) that is applied to any dice check that uses that skill.

Each skill is linked to an attribute, such as Physicality being linked to Unarmed Combat. Modifiers are cumulative and so dice checks include both AMods and SMods.

TABLE 3: SKILL MODIFIERS

CONDITIONAL MODIFIERS (CMods)
There are multiple factors that can influence a dice check, and these variables are represented by Conditional Modifiers (or CMods) that are added or subtracted from dice checks.

These CMods range from -5 (Doomed to Failure) to +5 (Divine Intervention) and are applied at the discretion of the GM or at the request of a player (see Making the Case). These can be wide-ranging and reflect the variety of elements that make up any given moment, such as frost making a wall harder to climb.

TABLE 4: CONDITIONAL MODIFIERS

OUTCOMES
A total score of 9 or above is a Success but different scores bring gradations of success.

TABLE 1: OUTCOMES

DIRE FAILURE
A score of 3 or less after all modifiers are applied is a Dire Failure. The character not only fails at the task or action, but also suffers some kind of setback or consequence that impacts them (for example, a failed safe-cracking attempt irreparably jams the lock, preventing further attempts).

FAILURE
On a score of 4 to 8, the action or task Fails (attempting to pick the lock on a safe is unsuccessful).

SUCCESS
On a score of 9 to 13, the action or task is Successful (attempting to pick the lock on a safe succeeds).

WILD SUCCESS
On a score of 14 or higher (after all modifiers are applied) is a Wild Success. The character succeeds in the task or action and there is an additional positive result that benefits the character (the safe not only opens, but there is something of great value inside).

MOMENT OF HIGH AND LOW INSIGHT
Characters can have a Moment of Insight when they perform exceptionally well or disastrously badly, giving them a deep understanding of why they succeeded or failed.

A double one is a Moment of Low Insight, which counts as a Dire Failure, in addition to the character receiving an Insight Dice.

A double six is a Moment of High Insight, which counts as a Wild Success, in addition to the character receiving an Insight Dice.

INSIGHT DICE
Insight Dice are a tool that empowers a player to affect and propel the Narrative directly.
Characters each start the game with 2 Insight Dice and receive an additional Insight Dice each time they get a Moment of Insight.

Insight Dice can be used in any way that a GM and player agree upon, but common uses are:
Adding an additional dice to the pool prior to making a dice check, for a 3d6 check
Adding +3 Conditional Modifier (CMod) to a dice check before the dice are rolled.
After a dice check, a player can remove one or both of the original dice and replace them with an Insight Dice, which are then rolled fresh and added to the total.
Insight Dice can be exchanged for a missing piece of required equipment, clue, or whatever the players can successfully Make The Case for
Players can give up all their Insight Dice to recover 1 Wound Point and 1 Resilience Point per dice and save their character from Dying. Players cannot choose how many dice to use in this method, it costs all of their available Insight Dice

Once an Insight Dice is used it is surrendered back to the GM.

Insight Dice cannot be transferred from character to character.

Insight Dice carry over from session to session.

FILLING IN THE GAPS

Filling In the Gaps is a GM tool that has the group help with world building and stage setting.

GMs should prompt players to provide additional context and Fill in The Gaps during gameplay about what their character is seeing or experiencing.

MAKING THE CASE
Similar to Filling in the Gaps, a character may also Make The Case and provide additional context how their actions are affecting the outcome of a dice check. If the GM agrees that what they are saying makes sense, they might add a CMod to the dice check, decided at the GMs discretion.

ATTRIBUTE CHECKS
Players can use a character's attributes for checks that don't require a specific skill, such as arm-wrestling or puzzle-solving. In these situations, only the AMod is applied to any dice check.

GROUP CHECKS

Multiple players attempting to undertake the same task can work together to increase their chances of success.

To take part in a Group Check, everyone must be using the same attributes or skill (even if they have a 0 in that skill). The player with the highest relevant AMod or SMod makes the check and they apply any AMods or CMods the other characters taking part in the check have.

Insight Dice cannot be used as part of a Group Check, although if the outcome of the dice check is a Moment of Insight, all members taking part receive an Insight Dice.

Group Checks are at GM discretion and must make logical sense within the narrative.

OPPOSED CHECKS
When multiple characters are directly involved in a task against each other, they make an Opposed Check.

Unlike all other checks in the game, there are no degrees of outcome with an Opposed Check. The result is based purely on the total score of each participant.

In the event of a draw, neither side gained the upper hand, and the check might need to be re-rolled.

PERCEPTION CHECKS
If a GM wants to know if a character is aware of subtle details or has picked up on subliminal queues, they may request a Perception check. This uses the Secondary Stat, Perception.

FIRST IMPRESSIONS
The first time a character meets a non-player character (NPC) they should make a First Impressions check to see the impression they make. This dice check uses Influence, Inspiration, Intimidation, or Manipulation, and the result determines future interactions.

On a Success, the NPC has no strong feeling and there is a 0 CMod to all future interactions.
On a Failure, they made a terrible impression and get a -1 CMod to all future interactions.
On a Dire Failure or Moment of Low Insight, they came across as threatening or hostile and get a -5 CMod to all future interactions.
On a Wild Success or Moment of High Insight, they made a favorable impression and get a +1 CMod to all future interactions.

GUT INSTINCTS
Characters can make Gut Instinct checks when they first meet NPCs to see what kind of an impression they get from this person. Characters can also make a Gut Instinct check when talking to an NPC to see if they believe what they are being told. This is an Acumen, Psychology*, Survival, or Tactics* check with the results determining what a character feels.

On a Success, the player gets some insight from the GM into how the NPC is coming across.
On a Failure, the player gets no good read on the NPC.
On a Dire Failure or Moment of Low Insight, the player takes everything the NPC says at face value.
On a Wild Success or Moment of High Insight, the player feels like they understand the NPCs motivations.

NEGOTIATIONS
Negotiations take place between players and NPCs and involve a process of Gambits and Rebuttals.

GAMBITS
The side leading the Negotiation makes an opening Gambit. This is an Influence, Barter, Inspiration, Intimidation, Manipulation, Psychology*, or Tactics* check.

On a Success, the Gambit has gotten the other party intrigued and they get a -1 CMod to their Rebuttal check.
On a Failure, the attempt falls short, and the NPC gets a +1 CMod to their Rebuttal.
On a Dire Failure or Moment of Low Insight, the Gambit is unsuccessful, and Negotiations are over.
On a Wild Success or Moment of High Insight, the Gambit was so successful that the NPC gets a -3 CMod to their Rebuttal check.

REBUTTALS
The other side now makes a Rebuttal that determines how the Negotiations resolve themselves. A Rebuttal is an Influence, Barter, Inspiration, Intimidation, Manipulation, Psychology*, or Tactics* check.

On Success, the Rebuttal has potentially swayed the other side, who may well consider this offer.
On a Failure, the Rebuttal is poorly received, and Negotiations are over.
On a Dire Failure or Moment of Low Insight, the Rebuttal is so unsuccessful that the situation now becomes tense, possibly even hostile.
On a Wild Success or Moment of High Insight, the Rebuttal was so successful that the other side is very likely to consider the proposal.