Effects are temporary behaviors that can be added or removed from a character. Every EffectType scriptable object assets describes an effect and points to a prefab that will be instantiated when the effect is added to a character. These prefabs have some implementation of EffectBase at its root which, through its Pool property, gets access to the character it was added to.
The GenericEffect implementation of EffectBase can be used when an effect is only visual or if the effects logic can be expressed using character instructions. Attribute- and StateEffect modify a characters attributes and stats and can be useful for buffs and de-buffs. DamageEffect periodically sends a damage event to the character and may be used for a burning or poisoned status for example. Lastly the TimeoutEffect is an expanded version of GenericEffect that removes itself after some time instead of having to be removed be some outside logic.
One built in way to add an effect to a character is the EffectResourceValue which adds the effect when its value is full. Another would be using a GenericTriggerArea which has events for when a character enters or leaves its area. Since EffectType has an Add method that takes a character as its parameter it is possible to link it directly in the inspector.
The SoulsPoisoned DamageEffect gets added when the players SoulsPoison EffectResourceValue reaches its maximum. The resource is increased when something deals SoulsPoisoning ResourceDamage to the player. This kind of damage is currently only dealt by the sprout enemy which has a ticking DamageSender and is either reduces by just waiting or by using the SoulsMoss ConsumableItem.
SoulsBuffed is a simple TimeoutEffect buff that doubles the characters strength for a set time and then removes itself which is added by the SoulsBooster consumable item. SoulsSlowed reduces a characters movement speed. Both of theses use character instructions for their actual effect.