The Janus Vortex is a spatial anomaly that affected the crew of the USS Excelsior in the late 23rd century.
Effects
The effects of travel in the Janus Vortex mirror the original transporter accident that transported James T. Kirk, Leonard H. McCoy, Montgomery Scott and Nyota Uhura to the mirror universe in 2267. The two universes ran parallel in terms of the fact that both versions of the Excelsior were travelling across the vortex from opposite ends. From the perspective of the crew, their ship changed to the ISS Excelsior, their uniforms had changed and a scar appeared on Captain Hikaru Sulu's face. This suggests that at some point within the anomoly, both ships exist as one, and when they separate, only brain patterns exchange places rather than physical matter. (TOS game: Shattered Universe).
History
Upon arrival in the universe, Sulu quickly realised that the only way back to the standard universe was via the Janus Vortex that was now in Tholian space. To get there, the Excelsior had to contend with the mirror versions of Klingons, Romulans and Tholians, plus the ISS Enterprise-A under the command of Pavel Chekov, who had become increasingly suspicious of his comrade.
After a difficult journey, the Excelsior managed to switch back places with its mirror universe counterpart, that had been easily contained by Commander Pavel Chekov in command of the USS Enterprise-A. (TOS game: Shattered Universe).