An ensouled aviansie head is an item which can be dropped by aviansie. It is used to gain Prayer experience by using the level 78 Magic spell Reanimate Aviansie from the Arceuus spellbook. When a player reanimates an ensouled aviansie head, a reanimated aviansie will appear and grant 1,234 Prayer experience after being killed. Like its living form, the reanimated aviansie cannot be killed with melee.
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.
For an exhaustive list of all known sources for this item, see here