T-Rex Achievement in Halo: The Master Chief Collection

This Easter Egg can be found in the mission, The Maw.

During the Warthog escape sequence at the end of this mission, Cortana will eventually warn you of a large gap in the runaway. At this point, floor it off the ramp and veer the Warthog off to the right or left so that you land on one of the sides of the platform below. From here, you can jump to the top of the tunnel where you would have originally landed if you nailed the Warthog jump; you’ll see a flaming canister in this location. Simply walk into the open corridor next this canister and inside you will see the name “Rex” spelled out in blood along the wall. The achievement will unlock as soon as you step foot in this corridor.