Many attempts to roleplay NPCs in combat requires to get them to make “fight or flight” decisions. Coming up with formal mechanics to model this would be probably inelegant. However, some kind of mechanics, even if vague, allow players to employ other strategy than to kill or render unconscious. Here is an attempt to sketch this down.
Let's assume first that the NPC is committed to a combat, willingly or not. This is the realm of reaction rolls, PC coercion or plain storytelling. The decision to enter a combat is left out of this discussion. Let's also assume that self-preservation is the rational decision which may be resisted by a Will check. This check can be modified by circumstance as we'll explore in Step 3.
For each NPC or coherent group of NPCs engaging one of more PCs, determine these three important parameters:
What is the ideal end-state of this combat? – The end-state where a NPC doesn't believe that more can be gained.
What are acceptable outcomes for this combat? – Acceptable outcomes are objectives that, once met, makes the NPC more likely to disengage if things start to go wrong.
What is the most favorable unacceptable end-state for this combat? – An unacceptable end-state that a NPC will avoid at all cost. If the stakes are low, keep track only of the least bad unacceptable outcome.
The resolve to keep on fighting is a matter of Will. A failure of Will triggers a flight decision. Not all flight decisions are desperate, a flight can also be a cessation of combat because the NPC believes that the battle is completely won. The number in parentheses is the base modifier to the Will check.
The ideal end-state is reached (very unfavorable, -3).
The ideal end-state cannot be reached (unfavorable, -1).
Major negative shift in the balance of odds (neutral, +0). - Major wound, stun, crippling, new foes, fewer allies, hit HP/3 threshold, new affliction/distraction.
Event leaving an unacceptable end-state as the best-case scenario (very hard, -6).
This is here that the GM can color the check with whatever factors appears to be relevant. The final Will modifier can be built from an open list, or just waived in one stroke. In the heat of a battle, assess the situation and use the task difficulty as outlined in B345.
Here is a list of factors that I came up with which may not be completely obvious:
The NPC thinks that he/she can still achieved the ideal-end state (+4).
The NPC thinks that he/she can still achieve one more acceptable outcome (+2).
The NPC thinks that if the combat ends now, one or more acceptable outcome are already achieved (-2): It's better to walk away while we're in the black.
The NPC is dealing with pain (shock) and/or a major wound (-1).
The NPC needs trauma treatment to stop bleeding (-1).
Flight implies an unacceptable outcome (+10): Flight is impossible, and surrender isn't an option.
The NPC realizes cannot achieve more than 8 to hit, faces a net defense of 12+, or is unlikely to do penetrating damage (-2).
Bloodlust, rage, bad tempers should here be factors to help resist flight.
Three brigands are surprised by two PC horsemen riding into the night while they are stealing from a frazzled traveler. The PC, reading the situation easily, decide to intervene.
The brigands have a Will of 10. The brigands' motivations are the following:
Ideal end-state: The PC are robbed of their money.
Acceptable outcome(s): The money from the traveler is taken.
Most favourable unacceptable end-state : The brigands are caught.
Case 1: The traveler slips out and runs away. The brigands could be deterred from fighting since their original objective is no longer achievable.
Trigger: An outcome cannot be met -1
Factor: They still think that the ideal end-state is possible +4
Total : Will+3, or 13, or 84%
Case 2: A third horseman arrives on the scene and join the other PCs.
Trigger : A major shift in odds (+0)
Factor: They still think that the ideal end-state is possible +4
Factor: An outcome cannot be met anymore (-1)
Total: Will+3, or 13, or 84%
Case 3: It is clear that the horseman know how to use their swords, and have the height advantage. The only thing to do from now on is to avoid getting caught or worst.
Trigger : Event leaving a unacceptable end-state as only option (-6)
Total : Will-6, or 4, or 2% (why bother?)
Case 4 : Same as Case 3, but the brigands are surrounded and can't realistically expect to escape. Those PCs look like they are going for the kill (as D&D characters tend to do so they get XPs).
Trigger: Event leaving a unacceptable end-state as only option (-6)
Factor: Their backs is against the proverbial wall (+10)
Total: Will+4, or 14, or 90% (a failure would be an attempt to surrender, even if futile)
Case 5 : Same as Case 4, but one of the brigand is unconscious, and the one rolling the check is bleeding and disarmed. Those PCs look like they are going for the kill (as D&D characters tend to do so they get XPs).
Trigger: Event leaving a unacceptable end-state as only option (-6)
Factor: Their backs is against the proverbial wall (+10)
Factor: Bleeding, in need of first-aid (-1)
Factor: Lost an ally (-1)
Factor: Disarmed (-2)
Total: Will+0, or 10, or 50% (catch-22 situation, flip a coin!)