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 was announced at Halo Fest that players would have to find them within the levels, similar to Halo 3. They will be unable to be seen in Classic Mode.
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, the Unggoy's heads will explode in a shower of confetti with a headshot.
Boom skull
Once the skull is activated, explosion radius will increase two-fold. [2]