Demiplane of Time

The Demiplane of Time, or Demi-Plane of Time,  also called the Plane of Time, Temporal Energy Plane, Time Plane or the Temporal Prime, was a demiplane  of the Ethereal Plane.

Description
The Demiplane of Time was an extradimensional space that could allow travelers to move through time. Very little was truly known about this ethereal demiplane, and despite diligent research, there were still too many inaccuracies and contradictions from "first-hand" accounts to conclude what was true, and what was not.

There were several different ways to describe the Demiplane of Time. Some believed it was a pocket dimension set aside from the rest of the multiverse, whilst others believed it was a plane that overlayed the Prime Material plane. Although not much was known for certain, and many had differing views, it was generally agreed that the place was dark, and permeated with a blue-gray mist. Threads and lines flowed through the vast darkness, which some believed to be timelines of living beings. It was also generally considered to be very dangerous, and could age those who passed through it considerably.

Place in Traditional Cosmology
In traditional cosmology, the Demi-Plane of Time was a large dimensional demiplane that operated by its own rules. It appeared to be almost as large as the Demiplane of Shadow, and its temporal manifestations extended into its Border Ethereal. It was hostile to life, but housed a few native species, such as vortex spiders. The curtain that surrounded the dimensional plane was semi-transparent, but had shadowy shapes that appeared beyond it. It did not show up on Great Wheel cosmology diagrams, the same with other demiplanes, as the properties of the extradimensional space were constantly changing with time.

Chronomancers' Viewpoint
Many chronomancers believed that the Demiplane of Time, which they called the Temporal Prime, was a dimension in which timestreams could be observed and used to move through time. Timestreams were formed by currents in time-space, which themselves were formed by the lifelines of living beings. However, these were not always stable, and thus could be turbulent, forming temporal vortices, allowing travel from one reality to another. The plane looked like an infinitely large dark space, with only timestreams illuminating its vast darkness.

Laws
The Temporal Prime, naturally, temporally affected those who went to or from it. After coming back from the plane, many found themselves either younger or older.

Substances that were created could not permanently exist on the demiplane, as the magical laws that created matter were very weak there. As such, both arcane and divine spells involving some form of matter (such as fog cloud and create water), would be impeded.

Importance in the Realms
The Temporal Prime was known to many deities, sages, and wizards of the Realms, who generally knew it as the Plane, or Demiplane, of Time. The demiplane was highly important to those such as Elminster Aumar, the Seven Sisters, and Khelben Arunsun. Mystra had granted them powers to prevent chronomancers from meddling with the temporal demiplane, which could potentially destroy history. If a chronomancer severely interrupted the timeline of events in Toril, they would be quickly and efficiently destroyed by an avatar of a deity.

As it was in the deities' interests to keep history complete and unbroken, Oghma and Deneir bestowed similar supernatural powers unto their followers. The Oghmanyte and Deneirrath priests were said to be able to follow the path of chronomancers through the Plane of Time and reverse the potential damage. As such, chronomancers made very limited voyages through time in Faerûn, Kara-Tur, Zakhara, and Maztica.

Vortex spiders, denizens of the Time Plane, were known to have rarely appeared in Faerûn.

Rumors & Legends
No one that had entered the plane was known to return. However, there were rumors that time-mages, known as chronomancers had mastered the art of traveling up and down the cords of time.