Fade

“I was shown vast oceans, containing not water, but memories, drawn from the minds of dreamers. I drifted through frozen moments, like paintings, perfect in each detail.” ―Magister Callistus 
The Fade, known by the Dalish Elves--and some Humans--as the Beyond, is a metaphysical realm that is tied to Thedas and separated by The Veil.
(All information within this text is derived from various other texts and accounts found throughout Thedas. Only one group was ever know to travel into the Fade physically, but their secrets have been lost, for now)

Geography

“There is no geography in the Fade. Place and time are far less important than concepts and symbols.” ―Grand Enchanter Fiona 
The substance of the Fade is sometimes referred to as the "ether". 
In its raw form, the Fade is a twisted, frightening world of dark rock and raw Lyrium veins where it is always night and gravity affects nothing equally. Gaps between dreams are such void places.
The Fade may appear as something else entirely when shaped by dreams. Much of the Fade is split up into fiefs or demesnes belonging to the spirits or demons that live there, and they change the landscape of the Fade to emulate what they see in the minds of mortal dreamers. They copy locations, objects, people and concepts of the real world, often in a cruel or confusing way. These copies are nowhere near consistent, and fluctuate according to the movement of the dreamers. Spirits whose realms are flocked with visitors rise to rule great portions of the Fade, while memories and concepts forgotten in the real world slowly drift away back into the ether, the spirits who ruled them losing all potency

History

Chantry teachings  According to the Chantry, the Fade is a realm of primeval matter from which the Maker formed the physical world and all living beings. It is sometimes compared to a well of souls, with lyrium being its "emerald waters.". It was the first realm created by the Maker, populated with spirits, the first of the Maker's "children." Growing unsatisfied with them (as what they created was "fleeting, ever-changing"), he then created Thedas, separated from the Fade by The Veil and populated by mortal creatures. Many spirits grew jealous of these new children, and became what mortals call demons, craving and/or emulating their basest desires and sins.
The Chant of Light states that the Black City was originally the Golden City, seat of the Maker. An attempt by Minrathous magisters to physically enter the city blackened it with their prideful ambition, and their punishment was to be cast down as the first of the darkspawn, commencing the First Blight. When the Chant of Light spreads to all corners of the world, the Maker will forgive mankind and return, restoring the Golden City. 
Dalish Beliefs  The Dalish refer to the Fade as the Beyond and believe that it is a holy place that was once the home of the gods. They also tell that following Fen'Harel's deception before the fall of Arlathan, the gods now lie imprisoned in the Eternal City at the heart of the Fade while the Dread Wolf roams, gleefully feasting on the souls of the dead.

Tourism

“Demons do not create everything in the Fade. They set the stage, as it were, and we fill it with our own dreams and nightmares.” ―Wynne                                                                                                    People of Thedas visit the Fade mentally when they dream, even though they do not remember the time spent there. The notable exception are the dwarves, who do not dream. The Qunari also claim that they do not dream as humans and elves do. Entering the Fade is forbidden according to the Qun, and the Qunari are rarely encountered there, but evidence suggests that at least some of them do dream. Mages tap into the Fade when they cast spells and are able to remain conscious while traveling there. 
Travel physically in the Fade is more possible than had previously been suspected, though dangerous. If a rift can be opened, it may allow physical access into the Fade; under normal circumstances the passage is fatal, however.

Type

Dimensional plane