Ptolus is a sprawling campaign setting for Dungeons and Dragons 3rd Edition, published by Monte Cook under an Open Gaming License. It was published in hard copy by Malhavoc Press in partnership with Sword and Sorcery Studios.
Ptolus was based on Cook's own ongoing campaign, and thus much of his work on (for example) Arcana Unearthed and Arcana Evolved was inspired more or less directly by it. Two Malhavoc Press publications, The Banewarrens and Chaositech, were published before 2006 but are essentially the basis for the Ptolus campaign.
The main book, Ptolus: Monte Cook's City by the Spire, was published in 2006 to much fanfare. The book included a CD-ROM with additional materials such as maps, adventure supplements and a character generator. In addition, the first chapter of the main book was published separately as A Player's Guide to Ptolus. Those who pre-ordered the main book prior to June 30, 2006 received five free copies of the Player's Guide as a bonus.
The supplement Night of Dissolution was included on the CD-ROM with the core book, but was also published separately. Additional blog posts meant to entice pre-orders for the book were combined into Secrets of the Delver's Guild and published in 2007.
Additional Ptolus content was made available on Cook's website.