A failed Shadow Gate was the result when an attempt to create a gate leading to the Plane of Shadow failed in some way. Rather than take a being to that plane, it risked subsuming their flesh with shadow, either killing them or transforming them into a dark creature.
A being that dared walk through a failed Shadow Gate was taken—for one brief, stuttering instant—to the Plane of Shadow, and in that moment a kind of seed of shadow was implanted in their flesh. After traversing the failed gate, a heartbeat later, a change came over them. They felt a film of cold flow over their body, and their skin, clothes, and items all appeared blurred. At this point, the victim could either accept the change or resist it with a great force of will, but success was not guaranteed.
Those who accepted or succumbed to the change saw their flesh and matter subsumed by shadow. This itself had only a 50% chance of success, and of survival. Complete subsumption saw the being survive intact and gain a "shadow mantle", which they could call up once a day to become a dark creature for some ten minutes at a time.
However, incomplete subsumption, which occurred half the time, caused the victim's possessions, skin, flesh, and all soft tissues to turn to shadow and mist fleetingly away. Death came in seconds, unless there was a quick-thinking someone on hand to successfully heal the victim. Without this, only bare bones remained, with no sign of the cause of death.
Those who successfully resisted the subsumption emerged unscathed and unchanged.
The fortunate few who survived at least ten passages of a failed Shadow Gate, or remained in proximity to shadow and shadow-worshipers, were changed permanently into dark creatures, and their hearts irrevocably evil.
In the Year of Lightning Storms, 1374 DR, when erecting their false Temple of Mystra in Wheloon, Cormyr, the clerics of Shar tried to save time on travelling to the Dusk Lord's Passage in the Vast Swamp. They attempted to create a gate to the Plane of Shadow so they could travel there directly. However, whether due to their own inexperience or interference by Mystra, the effort was a failure, though this was unknown to them at the time.
The first two to attempt a crossing were Father Sambar and a temple guard. Sambar died horrifically, but the guard survived as a dark creature. Rather than destroy it, they made of use it to transform themselves, the temple guards, and duped petitioners of the temple like Halish. The skeletons of those who failed, about fifty in all, were left behind the Shadow Gate and heaped around the altar as sacrifices. Temple guards who survived became shadow guards, whether through multiple crossings or continuing association with the Sharrans.  The Sharrans termed it the "Mystic Gate".
Eventually, on Eleint 6, some adventurers serving Mystra uncovered the Sharrans' evil deeds and ruined their plots. The War Wizards of Cormyr arrived in a week to safely deal with the failed Shadow Gate.
The failed Shadow Gate in the false Temple of Mystra appeared as a curtain of darkness framed between two ink-black pillars. It was only just transparent, but it made all things seen through it appear murky and dark, even bright lights. The Sharrans left the skeletons of the slain heaped around their altar behind the Shadow gate, seeing them as sacrifices to Shar.
Those well versed in arcane or divine lore may have had heard stories of Shadow Gates that could whisk a being away to the Plane of Shadow, but which sometimes left them tainted by shadow, or else they never returned. All such stories spoke of the extreme danger of using one.
Although only one failed Shadow Gate is known to exist, the wording of the entry in Cormyr: The Tearing of the Weave suggests that others are possible. Furthermore, "failed Shadow Gate" suggests that normal Shadow Gates are possible, but it it not clear what properties these have. Presumably, they function as ordinary gates or portals to the Plane of Shadow.
- ↑ 1.0 1.1 Richard Baker, Bruce R. Cordell, David Noonan, Matthew Sernett, James Wyatt (2007). Cormyr: The Tearing of the Weave. (Wizards of the Coast), p. 16. ISBN 07-8694-119-7.
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 Richard Baker, Bruce R. Cordell, David Noonan, Matthew Sernett, James Wyatt (2007). Cormyr: The Tearing of the Weave. (Wizards of the Coast), p. 154. ISBN 07-8694-119-7.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Richard Baker, Bruce R. Cordell, David Noonan, Matthew Sernett, James Wyatt (2007). Cormyr: The Tearing of the Weave. (Wizards of the Coast), pp. 18–19. ISBN 07-8694-119-7.
- ↑ 4.0 4.1 4.2 Richard Baker, Bruce R. Cordell, David Noonan, Matthew Sernett, James Wyatt (2007). Cormyr: The Tearing of the Weave. (Wizards of the Coast), p. 15. ISBN 07-8694-119-7.
- ↑ Brian R. James and Ed Greenwood (September, 2007). The Grand History of the Realms. (Wizards of the Coast), p. 156. ISBN 978-0-7869-4731-7.
- ↑ Richard Baker, Bruce R. Cordell, David Noonan, Matthew Sernett, James Wyatt (2007). Cormyr: The Tearing of the Weave. (Wizards of the Coast), p. 119. ISBN 07-8694-119-7.