Brundleplane

The Brundleplane is a demiplane created by Jack Brundle to house his phylactery, and also just to chill out in sometimes. The main entrance is behind an anonymous-looking door in the ruined city of Stelistos near Grimshaw, but there's a hidden entrance under a grave marked "Jack Bundel" in the town of Bundelborg.

Time seems to flow significantly faster in the brundleplane than outside; the party encountered a knight within who had fought with King Oswalt I a century ago, but who seemed to think only five years or so had past.

In addition to the monsters Brundle installed to guard his dungeon, the brundleplane was inhabited by the brundlings, 1-foot tall creatures in Brundle's likeness, and some duergar Brundle had enslaved and forced to dig out his dungeon. The duergar were under a curse that allowed them only in certain areas; if they went past some boundaries, they would instantly explode.

Everything in the Brundleplane is slowly freezing to death after the party. Also the party lied to the duergar about the curse being lift.