Rex

Rex spelled in blood in Halo: Combat Evolved.

Rex is an Easter egg that has appeared in Halo: Combat Evolved and Halo 2. The egg takes the form of the word "Rex" written on a level's scenery. It was added by Bungie programmer Tyson Green as a reference to his username "Ferrex".

Halo: Combat Evolved

The Maw

The word "Rex" can be found in the level The Maw, written on a wall in blood.

1. Proceed normally through The Maw on any difficulty level, until you get to the huge Warthog jump/free fall.
2. At the ledge, drive the Warthog to the right, so you land on the side of the floating platform.
3. Enter the corridor and look to the right side. Spelled out in blood is the word Rex.

Halo 2

Outskirts

On Outskirts, the word Rex is spelled in rocks with a pool of blood and a Covenant Energy Sword is sticking out of the ground. In Halo 2: Anniversary's remastered graphics mode, Rex is spelled out in shotgun shells.

1. Get to the rooftops and go to the area where the Jackal Snipers and Drones are.
2. Keep going until you reach the building with two snipers in it.
3. Turn and there should be a room in a building you can jump into, the Rex is in there, along with the sword. Keep in mind though that the Rex is not always in there and when it is, the sword may not have infinite energy. Rex does not appear in Co-op.

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  Rex spelled in rocks in Halo 2.

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  Rex spelled in shotgun shells in Halo 2: Anniversary.

The Great Journey

On top of the control room in the level The Great Journey, the word "REX" is written in Flood blood. It seems that the only way to see this particular Rex is by using Dev Cam mode, unless you have a pile of Plasma Grenades, a Type-32 Ghost, and you have the Sputnik Skull turned on. A video of how to get it can be found here.

Trivia

In Halo: The Master Chief Collection, finding the Rex on The Maw will unlock the T-Rex achievement, while finding the Rex Sword in Outskirts will unlock the Rex Sword achievement.