The Delta Triangle was a mysterious region of the Milky Way galaxy, which from the earliest days of space travel had been known as a navigational hazard. Starships reported inexplicable sensor fluctuations within the region, and a number of ships and species went missing in the area over the years.
In 2269, the USS Enterprise investigated the Delta Triangle, and discovered a "pocket dimension" called Elysia, in which many missing ships had been trapped. Among the ships trapped was the early warp vessel Bonaventure. A number of species that became trapped within the region -- including Andorians, Tellarites, Klingons, Orions, Vulcans, Romulans, Humans, Phylosians, and Gorn -- joined peacefully to form the Elysian Council.
After having engaged in battle, Captain James T. Kirk joined with Klingon Captain Kor of the IKS Klothos to escape this region. In later years, Kor would stress the battle aspect of this encounter. (TAS episode: "The Time Trap", TLE novel: The Art of the Impossible)