From Halopedia, the Halo wiki
Walkthrough: During the level Reclaimer after the Librarian cutscene. Exit the Librarian's chamber like you normally would, but instead of heading straight for the elevator, take a detour down the corridor on the right. The Terminal should be in plain view.
Note: Halo Waypoint names Terminal Three as "Charum Hakkor" and Terminal Four as "Flood". The subject matter of each terminal makes it clear that these titles were erroneously swapped at some point in the game's production. The article restores the titles in accordance with 343 Industries' original intent.
Forthencho, now aged after decades of war with the Forerunners, lies in a bed.
Shot of a door. The door explodes inward, and Forerunner Warrior-Servants storm the room beyond. They proceed to destroy the defending human forces, as the Didact narrates.
Shot of a large Forerunner fleet in orbit around Charum Hakkor. The camera pans right, revealing the wreckage of human ships and stations. The Didact strides down a passage. Warrior-Servants float to the sides, and human bodies litter the floor. Didact enters the bridge, where the final human resistance shelters behind energy barriers, rifles pointed at the door. Front and center stands Forthencho and one other soldier.
Didact's troops train their weapons on the humans. Forthencho places a hand on the gun of the soldier next to him, and forces the barrel downward. The camera pans down across the vast cityscape of Charum Hakkor, and then to a bunker below ground, where row upon row of human wounded and dead are laid out on beds. Forerunner machines float between rows.
Didact enters a research lab. The Librarian is there, looking at a stored Pod infector.
The Didact leaves the room.