The Master Chief Collection includes 37 different skulls, including several unique additions to Halo 2: Anniversary. With the exception of the pre-order skulls (which will unlock globally on December 12, noted with an *) and Swarm, every skull is unlocked and available from the start. Skulls are broken down into two categories: Scoring and Non-Scoring. Scoring skulls amp up the challenge and reward players with higher score multipliers or bonuses to base enemy values when turned on. Non-Scoring skulls can either be neutral, such as Grunt Birthday Party, or in cases of cheat-like skulls, have a score multiplier of zero. Zero multiplier skulls like Scarab disqualify level times and scores from being recorded on the leaderboard and disable achievements.
Here is a full run down of the entire skull list along with the games each skull appears in: