A Ferocious barbarian spirit appears as a result of using Mangled Bones at pyre site around Otto's whirlpool, south of Barbarian Outpost.
This monster will attack you when you have any type of log in your inventory, mangled bones, and click on the "Build pyre" spot along the edge of the lake in front of Otto the barbarian's hut. Protection prayers are recommended; this monster can hit fairly hard for its level, and cannot be poisoned. It is weak to air spells, so the polypore staff is effective. This monster does not drop dragon full helms. These can only be obtained as a direct drop from Mithril dragons, or by building a pyre ship with chewed bones, which are also obtained from killing Mithril dragons.