Playing in Ravenloft, if my Sorcerer tries to Banish one of the Darklords, what would happen? I know the Campaign Guides say the Darklords will always be returned to their demiplanes, but will they even be allowed to leave their jails for a moment?
Using just the available D&D5E resources for Ravenloft, consult the section in Chapter Two of Curse of Strahd on Alterations to Magic (emphasis added):
The land of Barovia resides in its own demiplane, isolated from all other planes, including the Material Plane. No spell — not even wish — allows one to escape from Strahd’s domain. Astral projection, teleport, plane shift, and similar spells cast for the purpose of leaving Barovia simply fail, as do effects that banish a creature to another plane of existence.
While the passage is specific to Barovia, because that's where the adventure book is set, it worked the same way in previous editions for all the Domains of Dread. The Dark Powers decide who comes and who goes, and they would never let a mere mortal send a Dark Lord anywhere but his custom prison.