Ymir's Skull

From the introduction:

When the gods made this world from the corpse of the colossal being Ymir, they left his mountainous skull intact as a memorial, hidden among a maze of forbidding peaks. For millennia intrepid heroes have attempted to delve it, and died. Several of their undecayed magic items, most very ancient, litter each cavity. The eye holes and nostrils gape; the teeth are shut tight.