Heretic Skull is an Achievement in Halo 3 and Halo: The Master Chief Collection. It is gained by finding the hidden skull on the Halo 3 Multiplayer map Heretic. Its symbol is the Mark of Shame, a mark which was branded upon the current Arbiter's body in the opening of Halo 2.
Go to the sword platform and face the two windows, now look to your right, on the right wall of the level, in height with the platform there are two "shutter" structures in the wall, and the skull is located on the bottom of the right structure. To get to the skull, simply enter monitor mode and fly over to the skull, go back to normal mode, and grab it before you fall too far down.
This is also one of the skulls needed for the Vidmaster Challenge: Brainpan. Its solitary Achievement is also worth 25 Gamerpoints, similar to the five other skulls of the Mythic Map Pack.