The Ninth Circle

The Ninth Circle is a bar inside of Underworld Concourse which is in the Museum of History. The bar is owned by Ahzrukhal; it is rivals with another store in Underworld.

Unmarked quest: Hired Help
Here, you can buy a contract for a "slave" named Charon for 2,000 Bottle Caps (or 1,000 Bottle Caps if your Barter is high enough) from Ahzrukhal; you can also get the contract by killing Greta in Carol's Place for Ahzrukhal.

Trivia
The Ninth Circle is most likely a reference to the 9th circle of Hell (aka the Underworld), from Dante's Inferno. The fact that a character named Charon appears in the Inferno to ferry Dante and Vergil across the Acheron river further suggests this connection. In Dante the Ninth Circle of Hell is reserved for traitors and betrayers.

It might also be a reference to Clive Barkers "Mister B. Gone" novel. It is the underworld childhood home of the book's daemon narrator Jakabok Botch. Jakabok fell into a fire as a child and was horribly burned and mutilated which when describing how he now looks is very similar to ghouls.