Below is a list of skulls that you can turn on (they do not need to be unlocked) in Halo: Reach:
Black Eye: You can only recharge your shields by using melee attacks on enemies.
Catch: Opponents throw grenades twice as frequently, and they go twice as fast.
Cloud: No motion sensor is available.
Famine: Half as much ammunition is dropped with weapons.
Iron: This sends you to the last checkpoint when a player dies in co-operative mode. In single player, you have to begin the whole mission from the start if you die.
Mythic: Enemies have twice as much health.
Thunderstorm: Enemies have more health.
Tilt: Enemy weaknesses and strengths are raised, making Elites only vulnerable to Plasma Pistols.
Tough Luck: Opponents will not retreat and will dive out of the way of attacks, Grunts will suicide attack you, and Brutes will charge at you.
Blind: No HUD (screen indicators), arms or weapon shown on screen.
Cowbell: Items are moved three times as far from explosions.
Grunt Birthday: Grunts explode if killed with a headshot, complete with confetti.
IWHBYD: Uncommon dialogue is swapped with common dialogue.
Added by: MongooseGeneral Aug 7th 2011, ID#7608