An ensouled kalphite head is an item which can be dropped by kalphites. It is used to gain Prayer experience by using the level 57 Magic spell Reanimate Kalphite from the Arceuus spellbook.
The head can be reanimated anywhere across Gielinor, so long as 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.
When an ensouled kalphite head is reanimated, a reanimated kalphite will appear and grant 884 Prayer experience after being killed.
For an exhaustive list of all known sources for this item, see here