The Decapitated head is the severed head of the Ice Troll King, whom players slay as part of The Fremennik Isles.
Players must decapitate the Ice Troll King after killing him - they do not gain the head in their inventory automatically. They should then take the head to Mawnis Burowgar in Neitiznot.
Unusually (even for a quest item), players cannot deposit this item in a bank. If the head is lost prior to completion of the quest, another can be obtained by reentering the Ice Troll Caves. In this scenario, upon entering the caves the player will be returned to the location of the Ice Troll King's corpse without having to pass through the first part of the cave.