Demiplane of Dread

From Forgotten Realms Wiki

The Demiplane of Dread is unique, similar to the plane that contains the city of Union in that many worlds and planes connect to it but it is still independant of them all.

The Demiplane of Dread is controlled by the relatively unknown Dark Powers and sections of the demiplane are ruled over by Dark Lords.

In relation to the Forgotten Realms, Castle Spulzeer in Tethyr was sucked into the demiplane by a magical gem set in the hilt of the dagger Aggarath. Jander Sundstar suffered a similar fate after he swore revenge for his dead love and the Lost King of Cormyr, Gondegal has apparently been spotted on the plane.