Halo: Reach skulls

As with Halo 3, Halo: Reach has difficulty-multipliers in the appearance of human skulls. In addition, all of the skulls featured in Halo 3 are included in Reach. Just like Halo 3: ODST, skulls are no longer easter eggs in-game, and can also be accessed in the campaign menu without finding it.

Black Eye
The Black Eye skull causes the automatic regeneration of the player's shields to be disabled. The only way to recharge the energy shields is to kill an enemy with a melee attack or assassination.

Blind
The player's first person HUD, weapon and arms are hidden.

Catch
The frequency in which enemies throw grenades is doubled, while also always dropping two grenades upon death.

Cloud
The Cloud skull is a new skull featured in Halo: Reach. When the skull is on, the player's Motion Sensor is disabled, replacing the Fog skull in Halo 3.

Cowbell
Purely for comical purposes, the Cowbell skull increases physics effects three-fold. Explosions and melee attacks launch players and NPCs further than normal, but damage is not affected.

Famine
All weapons have reduced ammunition or charge, much like the Halo 3 version of this skull.

Grunt Birthday
The Grunt Birthday Party skull causes headshots on Grunts to explode in a shower of confetti. However, this does no damage, unlike in Halo 3 and Halo 3: ODST.

Iron
If a player dies during the campaign for whatever reason, he/she is forced to restart the mission. On Co-op campaign if even one player dies everyone is forced to reset back to the last checkpoint reached.

IWBHD (I Would Have Been Your Daddy)
This skull activates rare and secret dialog throughout the campaign and turns off normal dialog.

Mythic
Enemies have twice as much health than they normally would.

Thunderstorm
All enemies' ranks are increased. Grunts and Elites are mostly promoted to Ultras with Zealots being much more frequent, Jackles spawn as Majors and Snipers, Skirmishers spawning as Commandos and Champions, Drones much more likely to be Majors or Ultras and Brute Captain Majors and Chieftains to spawn.

Tilt
The strengths and weaknesses of weapons are modified, with UNSC weapons and the Plasma Repeater affecting energy shielding more than health and visa versa for all Covenant weapons excluding the Plasma Repeater.

Tough Luck
Tough Luck is a skull where when activated causes enemies to dodge your grenades.

Firefight Custom Skulls
In Firefight, there are three custom made skulls you can have: Red, Blue, and Yellow. Each of these skulls has exactly the amount of customization ability as gametype modifications.