Halo 4 skulls
From Halopedia, the Halo wiki
Halo 4 skulls are game modifiers featured in Halo 4 with the appearance of a human skull. The skulls are the same from Halo: Reach. In addition, just like Halo 3: ODST and Halo: Reach, skulls are no longer easter eggs in-game, and can also be accessed in the campaign menu without claiming them in campaign levels.[1]
List of skulls
Famine
Weapons drop much less ammo.[1]
Tilt
Enemy resistances and weaknesses are increased.[1]
Mythic
Enemies have increased health.[1]
Catch
Enemies throw grenades more often.[1]
Black Eye
Energy shield don't recharge unless the player melee enemies.[1]
Tough Luck
Enemies always go berserk, always dive out of the way, and never flee.[1]
Iron
On campaign solo, player death results in the mission restarting. On cooperative play, player death results in reversion to previous checkpoint.[1]
Thunderstorm
Major upgrade to the capabilities of enemies.[1] Sangheili, excluding Zealots, will always appear as Sangheili Warriors. Unggoy will always appear as Unggoy Imperials. Kig-Yar will always appear as Kig-Yar Heavies. Promethean Crawlers will always appear as Alpha Crawlers. As with previous games, specialists, such as Rangers, are not affected.
Cloud
Motion tracker is disabled.[1]
Cowbell
Acceleration from explosions is increased.[1]
IWHBYD
Rare combat dialogue becomes more common.[1]
Grunt Birthday Party
Headshotting an Unggoy results in an explosion.[1]
Blind
The HUD and the first person arms and weapon are hidden.[1]