|This article is about the current version of DF.|
A soul is the receptacle of a creature's mental attributes, personality, memories, and thoughts. Although it is theoretically possible for a single creature to have multiple souls, creatures usually have at most one.
Souls are important to understand how various creatures behave: creatures with no soul are incapable of forming relationships, learning, or indeed doing anything at all beyond basic movement and fighting. Currently, these are undead that are
[OPPOSED_TO_LIFE]. Note that intelligent undead do have souls, even if they are impervious to emotions: dead citizens of your fortress raised as intelligent undead will become part of your fortress again, but NPCs killed by your adventurer will be hostile if raised as intelligent undead.
Raising a corpse as a non-intelligent undead erases its soul: even subsequent attempts at re-raising it as intelligent will result in a blank-slate creature with no pre-existing links or loyalties, which can be exploited by your adventurer, if you want to raise lieutenants out of fallen enemies.