Megg

Jaime Griesemer, lead designer on Halo: Combat Evolved, gave his then girlfriend Meg (aka Pallor of Subnova) a gift in the form of an easter egg. The egg, affectionally called the Megg, was hidden in Pillar of Autumn (Level) under a series of bizarre triggers (almost impossible feats) to ensure that no one could find it accidentally. The Megg itself is a heart made from human blood, with an M made from bullet holes in the center.

Directions

 * 1) Start the level on Legendary, single player.
 * 2) After you come out of the cryo-tube, turn and jump onto the top of it.
 * 3) Jump onto the shield-charger.
 * 4) Jump onto the walkway.
 * 5) Walk toward the wall with the exit.
 * 6) Walk the other way, to the other side.
 * 7) Jump onto the yellow canister.
 * 8) Wait ten seconds, and then jump back to the walkway.
 * 9) Walk toward the far wall.
 * 10) Step off, and then move through the level as normal until you reach the second battle.
 * 11) Run past the Marine who asks you to follow him.
 * 12) When you get to the bridge, run to the other side without triggering the cinematic and jump down into the small pit.
 * 13) Wait for the message about how to climb ladders to appear, and then climb up and trigger the cinematic.
 * 14) After Keyes gives you his Pistol, go through the exit and kill the three Grunts.
 * 15) Get the Assault Rifle and go back to the bridge.
 * 16) Kill a crewman with a headshot or melee.
 * 17) Run towards the exit.
 * 18) Invincible Marines will pour out of a previously locked door; use the AR to keep them at bay.
 * 19) As Cortana finishes saying "The Master Chief has gone rampant," a door in the corridor the Marines came from unlocks.
 * 20) Go through the door and enter a hallway. At the end of the hall, you will find the Megg on the ceiling.

Halo 2
There is also a Megg egg located in Halo 2. Discovered by decompiling the game script by Soldier of Light from High Impact Halo, Halo 2's version turned out to be much less complicated, while no less impressive. It is activated by going through the first level, Cairo Station, on Legendary, without losing your shields once. Upon reaching the final bomb room, a sound clip will play. The text of the clip is: "Megg, please report to the red courtesy phone. Megg, to the red courtesy phone."

Halo 3: Unknown