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.
As far as is known, the habitable areas of these planes consist of the interior of tremendous caves, far larger than are found elsewhere, reaching several kilometers in ceiling height in many cases. The Dread Dimensions are permeated with magic, to an extent even greater than Equestria, although much of it is dangerous and most of it is chaotic in nature. Light, and photosynthetic organisms, exist in the caves. Water heavily energized with magic emerges from fine cracks in the ceiling of the caves, where sessile magitrophic organisms produce visible light as a byproduct of their metabolism. This has the effect of illuminating the Dread Dimensions, although not very brightly, and the spectrum is more orange than noonday sunlight, resembling sunset. (Some areas of the Dread Dimensions are illuminated with other colors, produced by different species of magitrophs.) The light level is subject to periodic variation on a timescale of several hundred hours as magic-charged water flow varies.
Non-animal life also includes fungi and plants, some of which have been introduced from other planes of existence. The atmosphere and biology is compatible with Equestrian life, if not very friendly to it.
The Arcane Helix was banished (accidentally, by its own loremaster) to the Dread Dimensions in 704 AE, returning to Equestria in 1833 AE, this event and its aftermath being a major focus of PRPG1. The Terrors of Tambelon, a one-off PRPG testing session, may also have been set in the Dread Dimensions or a similar environment.
Leaders and States of the Dread Dimensions near the Arcane Helix
The Dread Dimensions are politically chaotic and prone to sudden changes in power structures, especially as compared to the notable stability of the Equestrian government. Is is generally ruled by the most powerful creatures who find themselves there.
The ancient phlegm dragon Lürax (Luerax) the Lubricious, herself banished from Equestria in the pre-classical period by a young Starswirl the Bearded, is a noted local power. Lürax loathes ponies more than any other form of life, but feared Dusty Tomes, as she believed the skeletal archmage to be Starswirl. Lürax rules great caves north and below the Arcane Helix. She does not trust henchmen very much. Her subjects are: six large, thriving colonies of Scales; her inbred dragon progeny (Lürax was producing fertile eggs when banished), including four children (adult Phlegm Dragons) and nine grandchildren (young Phlegm Dragons); a few grundle villages; and assorted other denizens of the Dread Dimensions who happen to live in her territory, not including any little ponies - that she has placed an ample bounty on these creatures is well-known, and she enjoys torturing them if captured.
Another power is Kingsley, Lion Queen of the Dread Dimensions, who once ruled a prosperous kingdom near Ponyland during the early classical period. A clever leader in battle, and harsh but fair in domestic policy, she made overtures of friendship to the ponies - but an accident during a state visit resulted in she and her entourage being carried aloft by hundreds of defective magical party balloons. She was believed to have died, but was instead carried to the Dread Dimensions through a rift in time and space, arriving almost two thousand years in the future. Beginning again as an adventurer and petty warlord, Kingsley worked her way back up to being Top Cat in a region of the Dread Dimensions just south of the Arcane Helix. Magic was mostly banned in Kingsley’s Kingdom, and unicorns were not allowed there unless horn-ringed by her soldiers. The rings were produced by Kingsley’s court mage, who is of course allowed to be un-ringed and is in charge of the magical defense of the kingdom, a rather weighty burden for one fairly skilled MAG 3 unicorn.
West of the Arcane Helix is a wasteland even by the standards of the Dread Dimensions, called the Wailing Wastes, but it is nonetheless the domain of the Shadhavar Prince Melu-Hurem VI (Wind Wail), a charming fellow who is a worshiper of the Almighty Slimefather.
Beyond the Wailing Wastes (further West) is the cloistered state of the immortal unicorn wizard Dancing Water of Unicornia, who provides a life of idyllic luxury for his followers by keeping a race of creatures known as Bushwoolies in magical bondage. Dancing Water was banished from Unicornia (an ancient state in the riparian valleys of the Unicorn Range) in 1520 BE, for having committed unspeakable crimes in his pursuit of immortality; he and his followers left to establish a “utopian” domain outside the reach of extant civilizations. Dancing Water is arguably quite insane. He is reaching the limits of his immortality, as well, and is beginning to deteriorate; already he has incrementally replaced the bodies of himself and many of his ancient followers with magical constructs. They are generally friendly to little ponies, but will be disinclined to let them leave. The ceiling above New Unicornia is covered with a selection of magitroph species maintained by gardeners to provide a reasonable approximation of natural light on Equestria; most of the towering structures have crystal roofs and indoor garden spaces. Magitroph lighting is supplemented by other magical lighting.
