An ensouled tzhaar head is an item which can be dropped by TzHaar-Ket. It is used to gain Prayer experience by using the level 69 Magic spell Reanimate TzHaar from the Arceuus spellbook reviving a TzHaar with 35 hp when used on the death location, granting 1,104 Prayer experience after killed. Killing the reanimated TzHaar will also grant 0.3% Arceuus favour.
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