The Rejarran portal was a massive, ring-shaped construct that enabled instantaneous one-way transport to another universe.
In the year 2308, the USS Excelsior encountered the portal in orbit of Rejarris II. The portal's guidance tractor beams ensnared the starship and attempted to drag it through its interdimensional interface. However, the Excelsior resisted but was destroyed when one of its warp nacelles smashed into the ring as it was being pulled through. Once the ship had passed through it, because of the damage it sustained, the portal returned to its base on the surface of Rejarris II.
While surveying the deserted Rejarris II in 2319, the crew of the USS Enterprise-B discovered the portal on the surface. After Commander Xintal Linojj lost her arm through the portal, and Captain Demora Sulu and Ensigns Hawkins Young and Galatea Kostas became stranded on the planet on the other side, the crew surmised that the Rejarrans used the portal to flee their world before it was impacted by an asteroid sometime within the previous half century.
Admiral John Harriman and Captain Amina Sasine arrived later in the skeleton-crewed Constellation-class USS Cassiopeia intending to travel through the portal to rescue the stranded Enterprise and Excelsior crews. The arrival of three Tzenkethi frigates cut the plan short and a new plan was made: The Cassiopeia crew transfered to the Enterprise which then flew through the portal. The portal was destroyed immediately afterwards when Harriman used the Cassiopeia's autopilot to crash into it to prevent both it and the ship from being captured by the Tzenkethi Coalition. (TLE novel: One Constant Star)