Halo: Combat Evolved Anniversary skulls
From Halopedia, the Halo wiki
The Halo: Combat Evolved Anniversary Skulls are game-modifiers that will be featured in Halo: Combat Evolved Anniversary with the appearance of a human skull. It is currently unknown whether the player will have to collect these skulls or if they will be available from the start, like in Halo 3: ODST and Halo: Reach.
Grunt Funeral Skull
Once the skull has been activated, every Grunt's methane tank will explode similar to a plasma grenade when killed. This skull will be available to those who pre-order the game.[1]
