Rex Sword Achievement in Halo: The Master Chief Collection

This Easter Egg can be found in the mission, Outskirts.

This hidden sword can be obtained right from the start of the aforementioned mission. Once you gain control of the Master Chief, exit the area of the Pelican crash site and head into the hallway. Kill the Grunt that is walking up the ramp and then turn around, jump on the light fixture above of the door, and perform a crouch jump onto the roof of the hallway. From here, you will need to jump to the raised rooftop to the left via another crouch jump.

Once you fall down unto the destroyed highway, perform a grenade jump to reach the roof of the buildings. Once on the rooftops, head towards the left and you will notice a ruined building with a dilapidated roof. Jump inside this building and you can see an Energy Sword sticking out of the ground, much like Excalibur. Once you retrieve the Energy Sword, the achievement will unlock.