Grunt Fiesta Skull: All Grunts speak Spanish and dance when they haven't spotted you.
Sissy Skull: Grenades and equipment can't be thrown as far.
Bad Day Skull: Brutes, Elites, and Hunters go into berserk mode easier and Grunts are more likely to go Kamikaze.
Methane Skull: If hit in the tank enough, the Grunt will begin flying around randomly until it explodes.
Bloody Mess Skull: GoW amounts of gore. Dismemberment and all.
Gangster Skull: 2x the amount of Covenant spawn.
Ninja Skull: All Covenant get minor Active Camo's for a short amount of time.
Cheat Skulls: Scoring and Achievements are disabled.
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DANGER DANGER DANGER! Skull: Start the map with a Scarab Gun.
Friendship Skull: All Covenant are friendlies.
That Was Too Easy Skull: Instant Kill
Try Your Worst Skull: Can't be hurt.
Ultimate Cache Skull: Unlimited ammo, grenades, and current equipment.
Sneaky Skull: Unlimited Max Active Camo.
Borderwatch Skull: Spawn with a magical Energy Sword called the Wabbajack. Anything vehicle, weapon, or enemy that's hit with it is turned into something else selected at random. Does not work on another player.