Mistreated in Life. Many house pets and familiars have terrible masters who mistreat the animals in life. When these creatures die (often as part of the master's mistreatment), night hags find the animals' souls and send them back to inhabit their bodies. The soul re-enters its old vessel, resulting in the creation of a yowler, an undead creature with supernatural powers, a burning desire for revenge on its old master, and a hatred of all living creatures.