An ensouled horror head is an item which can be dropped by cave horrors and jungle horrors. It is used to gain Prayer experience by using the level 52 Magic spell Reanimate Horror from the Arceuus spellbook. When a player reanimates an ensouled horror head, a reanimated horror will appear and grant 832 Prayer experience after being killed.
The head can be reanimated anywhere across Gielinor only if the spell is cast on the head while it is still on the ground. Once the head has been picked up, it must be taken to the Dark Altar in Arceuus to be reanimated.
It cannot be used as a replacement when on a cave horrors slayer task.
For an exhaustive list of all known sources for this item, see here