Black Room

The Black Room was a safe mode inside the Animus virtual machine, which could be accessed through a back door in its programming. The room was located below its polar opposite, the White Room loading screen. It was the "guts" of the Animus, as described by Subject 16. The Black Room was the original test program, which consisted of only basic physics and weather simulation. Not really intended for direct human intereface, the Black Room formed the backbone of the Animus' programming.

History
When Desmond Miles was placed back inside the Animus while in an unstable mental state in 2012, he used this room to look through his own lost personal memories. By accessing them, Desmond hoped to reintegrate the splintered layers of his subconscious and to wake up from his coma. However, Desmond was unable to wander out of the Black Room--doing so would have resulted in him being "deleted, like a llittle virus," according to Subject 16.

Somewhere in the Black Room, the key to Desmond's recovery from his coma was located at a point in time where the paths of Altaïr Ibn-La'Ahad, Ezio Auditore da Firenze, and Desmond all coincided, similar to a hub.

Trivia

 * The Black Room involves puzzle-oriented gameplay as a means of representing Desmond piecing together his subconscious mind. According to Alexandre Breault, this gameplay was inspired by the works of Rene Magritte and Roger Penrose.

Source

 * GameInformer, June 2011 edition