All monsters have an intelligence flag that by the game to determine certain behaviours. In general, a monster's intelligence is a secondary consideration compared to its physical and magical abilities, but it does have a number of effects, mostly in fairly specific situations.
Levels of Intelligence
There are five levels of monster intelligence:
- Plant intelligence: Plants, zombies, jellies, golems, elementals
- Insect intelligence: Insects
- Reptile intelligence: Reptiles, sea creatures
- Animal intelligence: Animals, some non-zombie undead
- Normal intelligence: Most humanoids, including lower-level spellcasters
- High intelligence: Higher-level spellcasters, some other powerful/magical monsters
Effects of intelligence
The following is a non-exhaustive list of effects of monster intelligence.
- Confused high intelligence monsters don't move if next to a deadly cell (i.e. drowning in water).
- High intelligence monsters get an extra +10 to hit when firing ranged weapons.
- Animal or greater level intelligence monsters blocking a chokepoint while fighting you will move to the side to let further reinforcements come in. [Source]
- Normal or greater level intelligence monsters will also move to the side if they are blocking a ranged ally further down a corridor from attack a target. [Source]
- Monsters with greater than insect-level int will avoid taking damage from slime walls.
- Monsters with normal or greater intelligence and a ranged attack will attempt to regain LOS if you move out of their vision. [Source]
- Monster memory: The higher the intelligence, the longer monsters will pursue you even if they lose LOS. [Source]
- Monsters smarter than an insect will retreat if they have LOS but can't reach you. [Source]
- Monsters must have greater than plant-level intelligence in order to go berserk.
- Higher intelligence monsters have an easier time spotting you if you are invisible. [Source]
- Dumb monsters (Animal or below) are more likely to be distracted by Projected Noise [Source]
- Dumb monsters (Animal or below) will not flee when low on HP. Intelligent monsters will.
- Friendly dumb monsters (Animal or below) don't turn hostile when damaged by ranged attacks.
- Elyvilon cannot pacify creatures of plant- or insect-level intelligence.
Prior to 0.13 reptile intelligence and insect intelligence were one value, and Elyvilon could thus pacify insects. Also, friendly dumb monsters wouldn't turn hostile when damaged by ranged attacks.