|This article is about a mod.|
Every creature that has a brain also has a soul. The soul is created as a separate item when the creature is butchered or slaughtered (or via a special reaction that directly affects unbutcherable corpses, for some races). Multi-headed creatures may produce multiple souls.
Souls are implemented as special plants; as such, they automatically wither after a few weeks (and can't be preserved in stockpiles). If you need a large number of souls for a ritual (or for several rituals), the best approach is to keep them in their creatures as long as possible, harvesting them immediately before they are needed.