From Halopedia, the Halo wiki
Easily accessed at Rally Point Alpha.
Go outside to the desert area, commandeering transport. You must complete a significant section of this level until you obtain tanks after defeating the Anti-Air Wraiths, then return in the tanks to the main door to the structure.
When you go inside the structure to let the tanks across the energy bridge, there will be a Terminal directly behind the controls for the bridge. You must activate the bridge before you can access this Terminal.
(Upon initial access)
(No message from Mendicant Bias upon shutdown)
(Upon being rerouted to new destination within Terminal)
(The following appears only if you access the Terminal on Legendary difficulty)
Note: (The Second AdjutantReflex avatar is seen during this message.)