From Halopedia, the Halo wiki
The Nexus is a Forerunner facility that lies directly beneath the Command Spire. It serve as a control node, directing the Reformation in conjunction with the Command Spire. The Nexus also serves as the main point of entry into the Command Spire itself, via a gravity lift.
The Nexus's central chamber is located deep within the Ring's superstructure. In order to reach the central chamber, one must first enter the Nexus through a door at the base of the Command Spire. They must then locate and activate a piston, exposing a long shaft which they must drop down through. They must then ride an anti-gravity gondola across a deep chasm and cross a light bridge before finally reaching the central chamber.
The entire Reformation Spire network, as well as a real time image of the Ring itself, appears in holographic form inside the central chamber. More importantly, the central chamber contains a gravity lift leading into the Command Spire. The power seeds needed to energize the lift's three Crucibles can be found in the adjacent chambers.
On December 13, 2559, when Cortana sacrificed herself to destroy the Silent Auditorium and deny Atriox control of Zeta Halo, a large section of the Ring broke off, taking with it several key Forerunner facilities including the Nexus. Months after her death, fragments of Cortana's data lingered in and around the Nexus, manifesting themselves as ghostly echos of various memories.
On May 28, 2560, the Harbinger extracted Despondent Pyre's Encephalon and used the Forerunner routines within to construct a protocol that summoned over forty Reformation Spires, including a Command Spire, around the Halo fragment. She then traveled to the Command Spire where she attempted to send the initialization sequence needed to fully activate the Spires. However, because the protocol she'd used to summon the Spires wasn't entirely correct, the Spire network resisted her attempts to start the Reformation.
Meanwhile, John-117 entered the Nexus in order to access the Command Spire and fought his way through Banished, Skimmers, and Aggressor Sentinels to reach the central chamber where he observed the Spire network waiting to be activated. The gravity lift connecting the Nexus to the Command Spire had been deactivated, so the Chief searched the adjacent chambers for the necessary power seeds. After fighting through more Sentinels, the Chief recovered the seeds and restored power to the lift's three Crucibles. During all of this, the Chief's AI companion, The Weapon, tried to get him to see the beauty and potential of everything around them. The Chief, however, insisted that all he saw was a problem. They also witnessed several of Cortana's memories play out inside the central chamber, eventually prompting the Chief to admit he still blamed himself for being unable to save Cortana.
When the Weapon accessed the Nexus's system to reactivate the lift, the Harbinger sprung a trap and hacked the Weapon, intent on using her AI routines in combination with Despondent Pyre's to break through the network's resistance and start the Reformation. The Weapon, in turn, used her link to the Harbinger to try to steal Despondent Pyre's encephalon. Despite the Master Chief's attempt to delete the Weapon - an effort which the Weapon herself stopped - both the Harbinger and the Weapon succeeded in their respective goals. This caused friction between the Chief and the Weapon, with the Chief being angry that the Weapon had stopped him from deleting her and had thus allowed the Reformation to begin, while the Weapon - who believed the Harbinger would have found a way to start the Reformation regardless - was angry that the Chief had tried to delete her in the first place rather than trust her.
With the Nexus's gravity lift back online, the Chief rode it into the Command Spire itself.
List of appearances