Rise of the Twilight Empire
Ghouls are what result when a corpse is infused with a demonic spirit of hunger as its animus. They possess a malign intelligence that allows them to lay cunning traps and plan ambushes. They are difficult to control, so most necromancers do not bother with them unless they specifically need a creature capable of complex planning beyond what an animate can offer.
When a mortal resorts to cannibalism in life, they sometimes open a door that allows the demonic spirit to possess their body after they die, and when the ghoul kills someone it opens a similar door for its brethren. Thus, anyone who is killed by a ghoul may rise again as a ghoul themselves.