What we know about the plan

140 ago, the Dusk Lord of Sessrendale was defeated and, according to legend might have escaped to the Plane of Shadow.

The letter from Esvele Graycastle to Despayr the black dragon indicates that they found the means to reopen the portal the Dusk Lord used, and urged Despayr to go through to the Plane of Shadow to make a new home. The Monastery of the Ebon Dome is apparently on the Plane of Shadow, along with a shadow image of the keep.

Starweaver Bestra’s quarters in the Lost Refuge included a statue of Queen Enchara, of Esparin, a kingdom absorbed by Cormyr over 200 years ago. The connection between Esparin and the keep (or the swamp) is unknown.

The torture and grisliness found throughout both the Lost Refuge and the fake Temple of Mystra in Wheloon — and the Tomb of Chonis, for that matter — is unusual for worshipers of Shar. They tend more toward subtle cruelties rather than mutilations and bloody sacrifices.

Despite the apparent existence of a portal between the temple and the Lost Refuge (used by Mhair), the temple still (apparently) required shipments from the Refuge, and the dominated petitioners were sent by sea and overland rather than using the portal.

What we know about the plan

FR Cormyr - The Tearing of the Weave LtPowers