Waterdeep:Skullport

Skullport, also known as the "Port of Shadows", is a dark reflection of the City of Splendors on the surface above. It sits on an ancient Netherese ruin in an immense cavern, and a network of catwalks and scaffolding create three layers of the town.

The Xanathar Guild is the de facto ruler of Skullport today, and the Xanathar's lair is accessible from one of the taverns in the town. However, this was not always the case. While it had always been largely lawless and run by gangs, prior to the Xanathar Guild taking over, all residents deferred to The Thirteen, a group of thirteen floating, flaming skulls which had once been mages from the Netheril empire in life. The Thirteen still float throughout Skullport, but their minds have apparently been lost, and what political power they might have held in the Port of Shadows has been ceded to the Xanathar.

Stronger than regular flameskulls, these ancient Netherese undead know spells that have been lost to history.