What are the 9 hells?

The Nine Hells, also known as Baator is the plane of lawful evil, home of the devils. The nine layers are Avernus, Dis, Minauros, Phlegethos, Stygia, Malbolge, Maladomini, Cania, and Nessus, and each had its own physical laws and properties of matter.

https://www.worldanvil.com/w/tales-of-faerun-autumnfyr/a/the-nine-hells-baator-article#:~:text=The%20Nine%20Hells%2C%20also%20known,laws%20and%20properties%20of%20matter.