An ensouled ogre head is an item which can be dropped by ogres. It is used to gain Prayer experience by using the level 40 Magic spell Reanimate Ogre from the Arceuus spellbook. When a player reanimates an ensouled ogre head, a reanimated ogre will appear and grant 716 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.
For an exhaustive list of all known sources for this item, see here