Tomb of Horrors
Buried inside Blackfire Hill in Skull City is the most infamous, most fearsome dungeon ever conceived.
Well, what’s left of it, anyway.
Formerly the original Tomb of Horrors, Acererak drained it of its eldritch and terrifying essence to power his plans for ascension. At the start of his latest scheme, Acererak used the flow of arcane energy through his tomb as a dimensional conduit linking his various interplanar dungeons and eldritch engines, feeding power from one to the other. When he returned and drained the magic from this place in one fell swoop, leaving it abandoned and also devastating portions of Skull City, those planar links remained. As such, although the bulk of the Abandoned Tomb is truly vacant, it does have several locations of planar crossover.