Some basic premises I’ll be starting from:
- Every creature wants, first and foremost, to survive. If it’s seriously wounded (by my definition, reduced to 40 percent of its maximum hit points or fewer—you may prefer a different threshold), it will try to flee. Exceptions are (a) fanatics or (b) intelligent beings who believe they’ll be hunted down and killed if they do flee.
- A creature with Intelligence of 7 or less operates purely from instinct. That doesn’t mean it uses its features ineffectively, only that it has one preferred modus operandi and isn’t going to be able to adjust if it stops working. A creature with Intelligence of 8 to 11 is unsophisticated in its tactics and largely lacking in strategy, but it can tell when things are going wrong and adjust to some degree. A creature with Intelligence of 12 or higher can come up with a good plan and coordinate with others; it probably also has multiple ways of attacking and/or defending and knows which is better in which situation. A creature with Intelligence of 14 or higher can not only plan but also accurately assess its enemies’ weaknesses and target accordingly.
- A creature with Wisdom of 7 or less has an underdeveloped survival instinct and may wait too long to flee. A creature with Wisdom of 8 to 11 knows when to flee but is indiscriminate in choosing targets. A creature with Wisdom of 12 or higher will choose targets carefully and may even refrain from combat in favor of parley if it recognizes that it’s outmatched. A creature with Wisdom of 14 or higher chooses its battles carefully and fights only when it’s sure it will win (or will be killed if it doesn’t fight).
- Physical abilities influence fighting styles. Low-Strength creatures, whatever their Dexterity and Constitution, will always try to compensate with numbers; if their numbers are reduced enough, they’ll scatter. Low-Constitution creatures, whatever their Strength or Dexterity, will prefer to attack from hiding. Low-Dexterity creatures, whatever their Strength or Constitution, will need to choose their battles carefully: since their ability to avoid damage is poor, they’ll want some sort of compensatory advantage. High-Strength, high-Constitution, low-Dexterity creatures are brutes that will welcome a close-quarters slugfest. High-Strength, low-Constitution, high-Dexterity creatures will use stealth and go for big-damage sneak attacks. Low-Strength, high-Dexterity, high-Constitution creatures are scrappy. Low-Strength, high-Dexterity, low-Constitution creatures will snipe at range. If all three physical abilities are low, a creature will seek to avoid fighting altogether unless it has some sort of advantage; if it’s intelligent, it may lay traps.
- Creatures that rely on numbers have an instinctive sense of how many of them are needed to take down a foe. Usually this will be at least 3 to 1. This sense is not perfect, but it’s accurate given certain base assumptions (which player characters may defy). The smarter a creature is, the more it will account for such things as its target’s armor, weaponry and behavior; the stupider it is, the more it will base its estimate solely on size.
- A creature with a feature that gives it an advantage (or gives its enemy a disadvantage) will always prefer to use that feature. If it can’t, it may even shun a battle altogether. On average, an advantage or disadvantage is worth approximately ±4 on a d20 roll; with midrange target numbers, it can be worth as much as ±5. It can turn a 50/50 chance into 3-to-1 odds, or 3-to-1 odds into 15-to-1 odds . . . or the reverse. Advantage and disadvantage are a big deal.
- A creature with a feature that requires a saving throw to avoid will generally favor this feature over a simple attack, even if the expected damage may not be as great. This is because the presumption of an attack action is failure, and the burden is on the attacker to prove success; the presumption of a feature that requires a saving throw is success, and the burden is on the defender to prove failure. Moreover, attacks that miss do no damage at all, ever; features that require saving throws often have damaging effects even if the targets make their saves.
- In Dungeons and Dragons, fifth edition, unless otherwise specified, any creature gets one action and up to one bonus action in a combat round, plus movement and up to one reaction. Any creature that exists in the D&D 5E game world will have evolved in accordance with this rule: it will seek to obtain the best possible result from whatever movement, actions, bonus actions and reactions are available to it. If it can combine two of them for a superior outcome, it will. I’ll refer to this principle as “action economy.”
- Alignment matters. Good creatures will tend to be friendly by default, neutral creatures indifferent, and evil creatures hostile—but lawful creatures, even lawful good creatures, will be hostile toward chaotic creatures causing ruckus, and nearly all creatures, regardless of alignment, are territorial to some degree or another. Intelligent lawful monsters may try to capture and either imprison or enslave characters whom intelligent chaotic monsters would simply kill.