Ptolus, subtitled "Monte Cook's City by the Spire", is a fantasy role-playing game campaign setting published by Malhavoc Press in 2006 that details a single city and the dungeons that lie beneath it. Ptolus uses the rules of the third edition of Dungeons & Dragons — the d20 System — under the terms of Wizards of the Coast's Open Game License. At 672 pages, it was the largest D&D supplement that had been published up to that time.