Nessus, detailed in many older D&D works, is the grim, barren home of Asmodeus. Each layer of hell has an archdevil ruler, which are (from Avernus to Nessus) Zariel, Dispater, Mammon, Belial and ...

The problem with Phlegethos is that, well, it just looks like Hell. With bursting volcanoes and hellfire everywhere, the ...