The Folding Halls are a series of interconnected extra-dimensional chambers. The known entrance is in the bottom of the lead bucket of Original Sin (Poison). This was in possession of the Vaakbonds within Depthsong Tavern before being gifted to Calliope by Don Tristurro.
The time inside of the Halls is different than on the Material Plane. Approximately one day passes in the Material Plane for each hour spent in the Halls, although the various chambers may not be consistent.