8th Terminal (Cortana Terminal)
I’m not sure who the first to discover this terminal was. The first time someone suggested that there was an eighth terminal in Halo 3, I simply brushed it off as a filthy rumor.
Turns out, there really is an eighth “terminal”. However, it’s not really the same as the others, and does not go towards the “Marathon Man” achievement.
Terminal 8 – The Cortana Terminal
The Eighth Terminal can be found on the level, “Cortana”, and is located on the basement floor of the room with the “Tilt Skull” in it. At the very bottom of this room, there is a Gravity Hammer leaning against a stand. This stand is a Terminal (although it does not look like the other terminals in the game, it simply looks like a pedestal of light).
Upon accessing it, Cortana will take over your HUD with a brief message, which is a reference to the Novel “The Fall of Reach”, according to Avateur over at ABO.