Halo: Reach skulls
From Halopedia, the Halo wiki
Halo: Reach skulls are gameplay-modifying difficulty-multipliers in the appearance of human skulls in Halo: Reach. Most of the skulls featured in Halo 3 are included in Halo: Reach. Just like Halo 3: ODST, skulls are no longer easter eggs in-game, and can also be accessed in the campaign menu without claiming them in campaign levels. Skulls, including the newly-introduced customizable Firefight skulls, are also present in Firefight, randomly activated on every beginning of a set. A campaign game on Legendary with all skulls activated is named "Mythic".
The Iron skull causes the mission to restart if the player dies during the campaign for whatever reason. On campaign Cooperative Play, if even one player dies everyone is forced to revert back to the last reached checkpoint. In Firefight, the skull disables respawning after the player died.
Tough Luck skull causes enemies to evade vehicles, dodge grenades and heavy weapon projectiles. Unggoy would be less likely to flee and could easily turn suicidal. Banshees are more likely to perform evasive maneuver when locked on by heavy weapons.
The Catch skull causes the frequency in which AI-controlled characters throw grenades to be doubled, while also always dropping two grenades upon death.
The Thunderstorm skull causes enemies' ranks to increase. Unggoy and Sangheili are, most if not all, promoted to Ultras, Kig-yar spawn as Majors and Snipers, Skirmishers spawning as Commandos and Champions, Yanme'e much more likely to be Majors or Ultras and Jiralhanae Captain Majors to spawn.
The Tilt skull exaggerates the strengths and weaknesses of weapons. For example, the magnum is only 1/4 as effective against energy shielding, while the plasma pistol is only 2/3 as effective against health.
The Mythic skull causes enemies to have twice as much health than they normally would.
The Blind skull causes the player's first person HUD, including the scoring box, weapon and arms, to disappear. The effect can be slightly compensated by pressing the "Back" button on the Xbox 360 controller, which gives a brief summary over the screen of the weapon and armor ability the player currently wields.
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.
Grunt Birthday Party
The Grunt Birthday Party skull causes headshots on Grunts to explode in a shower of confetti as well as producing a sound clip of children cheering. However, the confetti explosion does no damage, unlike in Halo 3 and Halo 3: ODST. The description is most likely a reference to the original Grunt Birthday Party skull in Halo 2, which will produce plasma grenade explosion on the Unggoy upon being shot in the head by the player.
This IWHBYD (I Would Have Been Your Daddy) skull causes rare and secret dialog to appear throughout the campaign and turns off normal dialog.
Firefight custom skulls
In Firefight, there are three customizable skulls the player can have: Red, Blue, and Yellow. Each of these skulls has exactly the amount of customization ability as gametype modifications have.
In Halo: The Master Chief Collection, the Cloud skull is renamed Fog to match the other games in the collection. When activated during a Firefight match, the on-screen text says "Fog" though the announcer still says "Cloud".