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]
Malfunction skull
Once the skull is activated, one random part of the HUD will disappear at every respawn.
Grunt Birthday Skull
Once the skull is activated, grunts heads will explode in a shower of confetti once headshot.
Boom skull
Once the skull is activated, explosion radius will increase 2 fold.
