From Halopedia, the Halo wiki
Flood is an achievement in Halo: The Master Chief Collection. It is awarded for activating the fourth Halo 4 Terminal on Reclaimer. The achievement is worth 5 Gamerscore and can be unlocked on any difficulty.
After the Librarian cutscene, exit the Librarian's chamber but instead of heading straight for the elevator, take a detour down the corridor on the right. The Terminal should be in plain view.