If you beat the game on Legendary mode, you will get a special, extended ending. Keep in mind that you only have to beat the final stage on Legendary mode, and not necessarily the rest of the game, in order to unlock this ending.Unlock the following Elite armor permutations by fulfilling the requirement listed:
Ascetic (Body) - Get the Up Close and Personal Achievement
Ascetic (Head) - Get the Steppin' Razor Achievement
Ascetic (Shoulders) - Get the Overkill Achievement
Commando (Body) - Get the Triple Kill Achievement
Commando (Head) - Get the Overkill Achievement
Commando (Shoulders) - Get the Killing Frenzy Achivement
Flight (Body) - Complete Tsavo Highway (on Heroic or Legendary)
Flight (Head) - Complete the campaign mode (on Legendary or Heroic)
Grunt Birthday Party (Located: Crow's Nest):
If you manage to get a headshot on a Grunt, the Grunt will explode.
Spartan Armor Permutations:
EOD (Body) - Complete Tsavo Highway (on Legendary)
EOD (Head) - Complete the campaign mode (on Legendary)
EOD (Shoulders) - Complete The Ark (on Legendary)
EVA (Body) - Complete Tsavo Highway (on Normal, Heroic, or Legendary)
EVA (Head) - Complete the campaign mode (on Normal)
EVA (Shoulders) - Complete The Ark (on Normal, Heroic, or Legendary)
Mark V (Head) - Get the UNSC Spartan Achievement
ODST (Head) - Get the Spartan Graduate Achievement
Rogue (Head) - Get the Spartan Officer Achievement
Scout (Body) - Get the Too Close to the Sun Achievement
Scout (Head) - Get the Used Car Salesman Achievement
Scout (Shoulders) - Get the We're in for Some Chop Achievement
Security (Head) - Get 1,000 Gamerscore Points
Security (Shoulders) - Get 750 Gamerscore Points
Gold Skull Locations:
Black Eye (Located: Crow's Nest)
When activated, you will have to battle enemies with melee tactics to regain shield energy.
Catch (Located: The Storm)
When activated, enemies will throw more grenades.
Famine (Located: The Ark)
When activated, dropped weapons will hold less ammunition.
Fog (Located: Floodgate)
When activated, the motion tracker disappears.
Iron (Located: Sierra 117)
When activated, you will be sent back to the beginning of the stage you're on when you die instead of to the last checkpoint.
Mythic (Located: Halo)
When activated, all enemies will have twice as much health.
Thunderstorm (Located: The Covenant)
When activated, enemies will be of a higher rank.
Tilt (Located: Cortana)
When activated, enemies will become more resilient to your attacks.
Tough Luck (Located: Tsavo Highway)
When activated, enemies are skilled at getting away from projectiles such as grenades.
While collecting skulls, keep in mind that you have to start the stage from the beginning (not from a checkpoint), but once you acquire the skull you're seeking, you can save and quit at any time without finishing the stage. This will allow you to collect skulls without having to worry about finishing the stage successfully.
On the second level of the campaign, when you're escorting characters to their ships, you can head the wrong away from where you're supposed to go to complete your mission. If you do, you'll run into two characters (separated by a door) having a humorous conversation.