A series of planes of alternate planes of existence, notoriously hostile, but highly magical. Equestria attempted to colonize them during the Golden Age. Many of these colonies were extirpated during the Catastrophe, and contact with any survivors has been lost.
The Arcane Helix was banished (accidentally, by its loremaster to the Dread Dimensions in 704 AE, returning to Equestria in 1833 AE.