The Antares Rift was a region of space located in the galaxy.
The Rift was a large phenomena which produced spatial disruptions that brought about minor damage to well shielded starships. It also made it extremely dangerous to use warp drive and forced vessels to continue at impulse speeds. The capabilities of the nebula were believed to be the reason many ships were lost in the region. The random nature of the extremely unstable rifts in the space time continuum meant that the region suffered from radically different physical laws. These energy fluctuations were also known to play havoc with communication systems. Each strike of the energy spikes were stronger with every strike and potentially could destroy a starship in the Rift.
Due to the fact that it was a different space time continuum present within the rift, ship sensors were unable to detect them. It was, however, possible to adjust the sensors in order to detect these spatial rifts in order to avoid them.
By the 23rd century, had gained a dangerous reputation as there had been numerous attempts at charting the Antares Rift all of which had vanished without a trace. The last few expeditions were done several years ago.
In the 2260s, the USS Regulus was going to be dispatched into the Rift on a mission to chart it but was diverted onto amother mission. Admiral Richards of Starfleet Command contacted James T. Kirk of the USS Enterprise and assigned him the task of journeying into the Antares Rift. Upon conducting the survey, the ship was forced to impulse speeds due to the spatial disruptions. During their mission, their ship was disabled by the Rift and they were boarded by Vurians. (TOS video game: Judgment Rites)