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.

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. Charon will follow you around and help you in combat.

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. 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.