He's Running a Marathon... Achievement in Halo: The Master Chief Collection

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

Take note that you must start this mission from the beginning on Legendary and have Classic Mode activated when watching the cutscenes. If these prerequisites are not in place, the hidden egg will not appear. After the initial opening cutscene between the Arbiter, Gravemind, and Master Chief concludes, the camera will pan to High Charity, where a mass of Grunts and Jackals are swarming the entrance to the inner council chamber. Here, you will notice a 2D figure of a man in boxers dancing within the Covenant crowd. Simply watch the cutscene through and the achievement will unlock naturally.