Tidbits from Fiends' Power Rituals

[Tobias Blackburn]

Hmm... I would guess that it would be whichever domain is closest. Unless the Dark Powers see a use for the fiend elsewhere.

Or, since it is such a unique situation, maybe it is now stuck in the western part of Barovia (since that was Gundarak) but is unable to cross the border into any other domain since it is the border of Barovia.

That could be interesting, since the fiend is forced to really consider how best to use that area, and has to create a power base without drawing Strahd's direct ire.