From publisher blurb:
Deep within the Sunken Fort is a stairway. This stairway leads into an unknowable and chaotic underworld: the Descent into Madness. Alien creatures, chaotic magic, formless order and untold treasures await the intrepid and the bold. They will not return the same as they entered.
This Mega-Dungeon is the continuation of the Sunken Fort. The Descent into Madness is a bunch of smaller dungeons cobbled together in a pocket universe connected to the normal world by a tenuous stone staircase.
The Descent is rumored to be a haphazard bridge between worlds that provides “safe” passage as opposed to tangling with extra-dimensional entities or sporadic magic portals that could leave you stranded. As a GM, you may use it to connect to wherever you like, or attach other modules to it as listed in this module.