Heading to his Destiny
From Halopedia, the Halo wiki
Heading to his Destiny is an achievement in Halo: The Master Chief Collection. It is obtained by finding the half-naked Bungie developer on the level Halo in Halo 3. The achievement is worth 5 Gamerscore.
Early in the level, there is a secret area for the 7th terminal. Next to the terminal's hallway, there are two rooms: to the left is a bright room with a vertical energy beam in the center; to the right is a dark room with a horizontal energy beam and a lethal chasm.
There is a small nook on the left side of the room (when looking out over the chasm). The half-naked guy is not visible from the door. As such, it is necessary for the player to propel themselves across the chasm and into the nook in order to view him and unlock the achievement. For methods of achieving this, see here.