An ensouled abyssal head is an Ensouled head which can be dropped by abyssal monsters. It is used to gain Prayer experience by using the level 85 Magic spell Reanimate Abyssal Creature from the Arceuus spellbook. When a player reanimates an ensouled abyssal head, a reanimated abyssal will appear and grant 1,300 Prayer experience after being killed. However, level 85 Slayer is required to harm the reanimated abyssal demon.
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