Why So Serious Achievement in Halo: The Master Chief Collection

To begin, load up a Custom Game on Coagulation and head towards Red Base. This hidden face can be found on one of the three large rocks located directly in front of Red Base. If you walk in the middle of these rocks and look towards the top half of the rock closest to the cliff in the distance, you should see the outline of a smiling face. The outline of the face may be a little hard to distinguish, so try looking at it from different angles until the achievement unlocks.