While often when running an encounter as the DM, the motivations of the party's opponents is pretty clear, sometimes when the encounter was randomly or somewhat-randomly put in there, or it's not a "key" encounter to the plot, it can be hard for me to figure out what the opponents' motivations are. In particular, it can be helpful to know when an opponent might turn and run or otherwise want to stop fighting.
Many years ago I played AD&D 2nd edition, and it has a mechanic called a "Morale Check" designed to help a DM figure this out. In true 2e style, it consists of a table (DMG Table 49) listing a base morale rating for certain types of creatures, and then a whole bunch of modifiers (DMG Table 50) based on the situation and more specific details about the creature. This gave each creature its "morale rating", and if circumstances start going badly for the creature, the DM can roll 2d10, and the creature will keep fighting if the result is equal to or below the morale rating, and try to leave the fight if the result is above it.
I want to do something similar in 5e, for those cases where I want a die roll to help me figure out when an opponent wants to stay, but I'm not really sure how to do it. I suppose I could just use the 2e charts outright, but I really like how 5e has done away with those huge lists of charts and I can probably replace a lot of situational modifiers by just deciding when to give disadvantage or advantage on a check. But if I do it as an ability check (which seems like the 5e approach to this sort of thing), I'm not sure what ability to use.
- Using a physical stat (Strength, Dexterity, Constitution) doesn't seem right, because I think I'm looking for something that's more of a mental willingness to fight, though I could see an argument for Constitution if I'm trying to model something that's more physical stamina.
- Intelligence doesn't seem like the best fit either, but it could work to model a logical reasoning and deduction of whether they're likely to be beat.
- Wisdom seems the closest from an intuitive understanding of what the word "wisdom" means, but 5e uses the term to mean something that's more about Perception and Insight than about making wise choices. Also, I don't know if I as the DM could easily figure out whether the wisest choice for the creature could be to continue attacking or to flee, when what I think I want is a way to have the dice make the decision for me.
- Charisma, particularly with the Intimidation skill, is kind of the opposite of what I want. Maybe I am thinking about this all backwards, and it'd make sense for the party to make some group Charisma check to decide whether the creature surrenders.
And with any of those, I don't know how I'd set a DC, or whether passing the check would mean "fight more" or "try to stop fighting". With all that reasoning in my mind, I figure I'm probably approaching this all the wrong way. What approaches do you find work to help determine whether a creature fights or flees when you aren't sure of a creature's morale? Is there a 5e mechanic that models this concept well, or that I at least don't have to twist too hard to make work for me?