Artifact Esmund Clockwright created on the behest of the Deeplurker to send it back to its home plane. Originally able to just open existing Doors wider and close them. Using further information discovered from the library and Maigran of alternate-Myrwood and the Stone Tower, Esmund was able to further refine it to teleport them between planes (when in an alternate reality) and to change where Doors lead.
(Maigran of alternate-Myrwood warned that such dimension-sliding was impossible, because the only way it would be possible is if there was such a catastrophic failure of the borders between realities that all dimensions were crumbling in a cascading failure state.)