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, affectionately 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 Captain 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.

Campaign
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."

Multiplayer
On Coagulation (Level) there is a Megg that can be viewed in game.

When you finish this go to blue base in Coagulation. Use the Spectre's boost to climb up the right part of the cliff near the base. There is a ridge on the wall as you continue left that forces you down, so stay clear of it by moving slightly down from it.
 * First, change your character model to an Elite, if it isn't already (this is needed because they get out of the Spectre from the front) and make a gametype where you have:
 * an overshield
 * a couple of Spectres.

By now you are completely sideways, and approaching a ridge on the cliff. keep boosting on it, and you'll eventually get over it. It may take a few tries but just be patient and keep trying.

When you get to the very top of the cliff, turn around in your Spectre, keep its left wing on the rim you originally approached, move the nose of the craft slightly over the rim, and search for a small notch in the cliff. I'm not entirely sure whether it was pure luck or timing that got me over it, but you have to get out of the vehicle and hopefully you will be over the level.

Now to find the sign. Walk along the cliff to red base but DON'T walk on the edge! There are notches in it that you can fall into and never get out of. Once at the back end of red base's cliffs, look around for a white patch in the rock. At first it won't look like anything, but back up and look again. There is a "M" clearly visible on the cliff.