Halo: The Master Chief Collection skulls
From Halopedia, the Halo wiki
Skulls are gameplay modifiers with the appearance of a human skull featured in Halo: The Master Chief Collection. Like in Halo 3: ODST, Halo: Reach and Halo 4, the skulls are no longer easter eggs in-game, and can also be accessed in the campaign menu without claiming them in campaign levels. A total 38 skulls are included in the collection, including several unique additions to Halo 2: Anniversary.
Skulls in The Master Chief Collection are divided into two categories: Scoring and Non-Scoring. Scoring skulls increase 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 score and time-based achievements.
List of skulls
The games in which the skull is available are listed below each skull. Pre-order skulls are marked with ¹ and unlocked globally on December 12, 2014. With the exception of these as well as the Swarm skull, all other skulls are unlocked from the start. Skulls marked with ² were not available in the original release of the games, but were implemented via updates in the MCC Insider Program. The Acrophobia skull was added into the game for Halo 3 for the release of Halo 3 on PC. The Acrophobia can only be unlocked for use by completing a seasonal challenge to kill 343 flying enemies in Halo 3, between July 14 and July 29, 2020. It was unlocked globally after July 29, 2020.