Halo 4 skulls

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 in 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.

Famine
Weapons drop much less ammo.

Tilt
Enemy resistances and weaknesses are increased. This also affects the player.

Mythic
Enemies have increased health.

Catch
Enemies throw grenades more often.

Black Eye
Energy shields don't recharge unless the player melees enemies. The following actions also recharge energy shields:
 * The RT attacks of the Gravity Hammer and Energy Sword
 * An enemy being in the Gravity Hammer's shockwave area of effect
 * Boarding/hijacking an enemy vehicle
 * Punching (not boarding/hijacking) an enemy vehicle, manned turret, or Shade Turret
 * Punching the Lich's core, in any levels that feature Liches
 * Deploying a Regeneration Field
 * Betraying allies and punching them

Unlike past Halo games, the low shield beep and shield down beep sound effects are removed when the Black Eye skull is active.

Tough Luck
Enemies always go berserk, always dive out of the way, and never flee.

Iron
On campaign solo, player death results in the mission restarting. On cooperative play, player death results in reversion to previous checkpoint.

Thunderstorm
Major upgrade to the capabilities of enemies. Sangheili, excluding Zealots, will always appear as Sangheili Warriors. Unggoy will always appear as Unggoy Heavies. 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. Promethean Knights and Promethean Watchers are also immune to the Thunderstorm skull's effects.

Cloud / Fog
Motion tracker is disabled.

Cowbell
Acceleration from explosions is increased. Melee attacks also increase knockback.

IWHBYD
Rare combat dialogue becomes more common.

Grunt Birthday Party
Headshotting an Unggoy results in an explosion and sounds of cheering people. The following weapons can trigger this effect:
 * Magnum
 * Battle Rifle
 * DMR
 * Sniper Rifle
 * Carbine
 * Beam Rifle
 * Boltshot
 * LightRifle
 * Binary Rifle

Blind
The HUD and the first person arms and weapon are hidden.

Trivia
In the level Forerunner, the animation of the Promethean Knight leaping onto John-117 is changed if the level is played with the Black Eye Skull on (except on Easy). In the alternate animation, the Knight shoots him after being kicked off to ensure the player's shields don't recharge after their melee.