An ensouled chaos druid head is an item which can be dropped by chaos druids. It is used to gain Prayer experience by using the level 30 Magic spell Reanimate Chaos Druid from the Arceuus spellbook. When a player reanimates an ensouled chaos druid head, the player will be granted 60 Magic experience, causing a reanimated chaos druid to spawn. After the kill, 584 Prayer experience will be granted.
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