A region of space where many starships have vanished, leading to a pocket universe and the Elysian Council in The Time Trap.
The Delta Triangle is a region of space known for the disappearance of many ships over the centuries. In The Time Trap, the Enterprise is exploring it when it is attacked by Klingon ships and pulled into a space-time vortex.
On the other side is a pocket universe called Elysia, where the descendants of crews from many lost ships have formed the Elysian Council. Kirk learns that the time warp will gradually destroy the ship's dilithium crystals, and the Enterprise and a Klingon ship must work together to escape.