The Orb Chamber is the final room of Amnesia: The Dark Descent. The Chamber lies at the very heart of Brennenburg Castle and is also the deepest room into the ground. It is Alexander's most precious and hallowed of all chambers.
The Chamber is accessible from the Inner Sanctum (of which it is also a part of). It was seemingly built for the sole purpose of allowing Alexander to return to his own world and, as such, is very utilitarian in this regard. Both the Orb Chamber and the Inner Sanctum are all but impenetrable while the chamber is guested by an Orb. Only the power of another can overwhelm the barrier around it, allowing entry.
The room is fueled by a massive steam engine and uses the power of the Orb to open a portal. The Orb's energy is channeled through three pylons containing the vitae taken from countless people tortured to death. These structures surround a center altar upon which the portal forms. When the ritual commences the Orb will tear a rip in space and time, an interdimensional portal that will allow one to pass from this world into another. It is here, at last, that Alexander is found: levitating nude in the air while performing the portal ritual. He is up on a raised platform, which is surrounded by blue flames that prevent Daniel from getting close to him. Behind Alexander, strange (possibly occult) writing can be found on a tall, slender stone. Up near the ceiling, there are numerous rocks suspended in midair circling the portal device.
Before the events of the Dark Descent, Alexander was fearful Wilhelm and his men would betray him to the Law, so he proceeded with the final ritual without proper foresight in using these relics. He accidentally cracked Agrippa's recovered Orb and it fell into six parts. Alexander tried everything, but was unable to mend it.
Later, at Alexander's response to his letter, Daniel fled London for Brennenburg hoping to escape the Shadow following him. Alexander convinced him that together they could banish this guardian forever in a ritual. This was a lie. The true purpose of the Inner Sanctum and Orb Chamber however had always been to tame the power of the Orbs, forcing them to yield. Daniel was unaware of this, putting all his hope and faith in the Baron liberating him from this ancient horror.
On the 8th of August 1839 Alexander took Daniel into the Orb Chamber and tampered with the relic. The invisible Shadow was angered and its fleshy residue materialized. Alexander covered the Orb to prevent it consuming him. Alexander told a naive Daniel the nightmarish force was closer than he thought. He used this excuse to get Daniel to cooperate with him in extracting and gathering vitae from the prisoners to complete his "ritual".
It is later revealed in a letter that Alexander knew, and even possibly regretted, the fact that he could not bring Daniel with him once the ritual was complete, even if they had acquired enough energy to allow two to enter. Alexander wrote that he found it ironic that he would have to do the same thing to Daniel that Agrippa and Johann Weyer had done to him years before. Alexander believes Daniel has been "branded" by the Shadow from the beginning, and that the Shadow follow Daniel wherever he goes. Worse, it is possible that Daniel becomes marked for death because of the baron's meddling. When Alexander has the ignorant young man gather the last of the vitae required, he steals his Orb and abandons him, leaving a furious and grief-stricken Daniel as an offering to a vengeful and bloodthirsty Guardian.
Eventually Daniel, who had already destroyed his memories with the amnesia mixture, arrives to confront the Baron. He finds and rebuilds Agrippa's Orb to remove the barriers to the chamber. Using his own blood he breaks the final seal placed on the doors.
Once inside Daniel can opt to do several things that will result in three different endings. If he allows the ritual to take place, either by doing nothing and watch Alexander escape or throwing Agrippa's head through the portal, the wrathful Shadow will consume him for abusing the artifact's power. Lastly, he can choose to avenge his former-self and the deaths of all the innocents killed by pushing over the three unstable pylons. This messes up the ritual. The satisfied Shadow will devour Alexander as atonement, but leaves Daniel alone and free to go with a guiltless conscience. The last scene in the game (after pushing the pylons down) is Daniel walking out of the castle, hearing the castle crumble behind as he leaves.
- Alexander states that the purpose of his machine is to emulate a Traveller's Locket, which allow the owner to travel through different worlds.
- The interior and walls look much older than the rest of Castle Brennenburg. It is possible that this chamber is a genuine Mithraeum, a place of worship in Mithraism, and the same one where Agrippa found his first Orb.