Cryptum is an achievement in Halo: The Master Chief Collection. It is awarded for activating the seventh Halo 4 Terminal on Midnight. The achievement is worth 5 gamerpoints and can be unlocked on any difficulty.
During the level Midnight, shortly after boarding the Mantle's Approach, it is in the third room with enemies in it. There should be no Knights here. The Terminal is located in this room's central structure.
The Cryptum and Heretic Skull and achievements in the Master Chief Collection utilize the exact same achievement art.