Herecit Skull

Heretic Skull Effect: The Effect of the Heretic Skull is unknown. There may be no effect whatsoever.
Heretic Skull Location:
The Heretic Skull can be found on the Mythic map, “Heretic”. To get the heretic Skull, enter Heretic in forge mode, then fly towards the top of the Blue base and the skull is located in the vent-like ducts.
You can see photos and a Heretic skull location video below===============================^

Hint 1: Heretic from the Halo 3 Mythic Maps. The Location of the skull is highlighted above.