Universal Entity Behavior

Entities or the Lost are not there to just make the player scream and run away, or fight - they are living beings. Living beings that clash with your survival. The game must show that they are actual living creatures - not just simple enemies

Universal needs

Every entity has similar systems to the player:

Same Trauma & Disease system
Hunger & Thirst
Fatigue & Tiredness

To survive an entity needs to keep their hunger and thirst up, to not be exhausted by sleeping well and to avoid getting trauma or diseases

EntityInstincts

This is done by Instinct Drives in the EntityInstinct component. Each drive has a pressure value that increases on certain conditions and slowly decreases over time:

Hunger Drive - Increases when the hunger gets lower. Thirst is equal to hunger. Grows linearly with hunger stat and doesn’t decay by itself
Fear Drive - Increases fast when the entity is Injured or in a situation that compromises their Health. Fear can also increase when seeing something in a room (Moving object, other creature of player) (IE “Startling”)
Fatigue Drive - Increases when the entity gets more tired as it performs actions like sprinting, attacking or just existing. Grows linearly with the fatigue of the creature and doesn’t decay by itself
Curiosity Drive - Increases when there are nearby points of interest. Danger rooms appearing, sources of sound, movement in their field of view
Aggression Drive - Increases when the entity is “frustrated”. Tied closely to other Drives. Controls how much the entity is ready to fight

Each drive has a stack of actions associated with it. The higher the drive pressure - the more extreme the action will be:

Pressure vs Drive	Hunger	Fear	Fatigue	Curiosity	Aggression
0	-	-	-	-	Ignore
0.25	Seek nearby Danger rooms.
(Easiest source of food)	Increase aggression, more alert	Instead of wandering from room to room, stay in current room and only leave if prompted	Crawl to closest point of interest	Signal to keep distance (Eg: Growl, stay low)
0.5	Seek nearby lesser entities (Anything smaller or weaker)	Keep distance from target	Seek room with darkness, no mold and least danger.	Quietly stalk to the closest point of interest	Attack if target is too close
0.75	Increase radius of searching for food. Lower fear	Stay away from line of sight of target	Choice to fall asleep if room is good and avoid sprinting for tasks	Walk to closest point of interest. Increase aggression	Attack target if in line of sight
1.0	Eat anything that can be eaten, even if it is the corpse of its own kind	Sprint to room with least danger and farthest away from target.
If unable - Convert to aggression	Force fall asleep	Sprint at full speed to point of interest	Attack everything in the vicinity, even its own kind

Instincts are not independent. There are relationships between drives:

Higher hunger - Higher aggression (starvation & desperation)
Higher fear - Lower curiosity (Loss of interest over risk)
Higher fear - Lower fatigue (Electrifying sensation of fear)