Adimarchus had two forms, and could shift between them at will. The first one was reminiscent of his past self as a planetar: a beautiful androgynous angel with pale purple skin, a pair of metallic golden wings, and black eyes. While similar in stature, his second form had ash-black skin and white eyes, and the wings were replaced by four shadowy tentacles ending in lamprey's maws. In both forms, Adimarchus' right eye burned with a smoking flame, a sign bestowed on any creature destined to rule Occipitus.
As the ruler of Occipitus, Adimarchus could draw tremendous evil power from the fabric of the layer itself. He could remake the landscape with a wave of his hand, and could control magic use across the layer. He had the power to create powerful mummy lords and to summon other tanar'ri and he had access to various spell-like abilities.
As the Demon Prince of Madness, Adimarchus had the power to plant fragments of his consciousness into the minds of distant creatures, even on different planes.
Adimarchus was a fallen planetar who once led a demonic army with the goal to invade Celestia. During that battle and in order to contain the invasion, the angels detached a portion of the plane and cast it into the Abyss, where it merged with the existing layer of Occipitus.
Adimarchus survived the battle, and became the ruler of the layer by incorporating the wreckage of Celestia into Occipitus, becoming a demon lord in the process. As a demon lord he often waged war against his most powerful rival, the demon lord Graz'zt.
During his reign, Adimarchus created the Test of the Smoking eye, a difficult challenge that would be used to find a suitable successor should anything happen to him.
After several hundred years of rule, Adimarchus was betrayed by Athux, the son of Graz'zt, and disappeared from Occipitus. He was imprisoned in Orthrys, the first layer of Carceri, where he was driven to madness. Some sages suggested that Adimarchus's deposition, together with Waukeen's imprisonment in Azzagrat in 1370 DR and his numerous other schemes, was a component of Graz'zt's plot to unify all demons of the Abyss.
- David Noonan (February 2004). “Test of the Smoking Eye”. In Erik Mona ed. Dungeon #107 (Paizo Publishing, LLC), p. 62.
- Christopher Perkins (November 2004). “Asylum”. In Erik Mona ed. Dungeon #116 (Paizo Publishing, LLC), pp. 61–62.
- Ed Stark, James Jacobs, Erik Mona (June 13, 2006). Fiendish Codex I: Hordes of the Abyss. (Wizards of the Coast), p. 65. ISBN 0-7869-3919-2.
- Mike Mearls, Brian R. James, Steve Townshend (July 2010). Demonomicon. Edited by Scott Fitzgerald Gray. (Wizards of the Coast), p. 58. ISBN 978-0786954926.
- James Jacobs (October 2007). “The Demonomicon of Iggwilv: Graz'zt, the Dark Prince”. In Kim Mohan ed. Dragon #360 (Wizards of the Coast), p. 13. Archived from the original on 2009-06-03. Retrieved on 2019-08-27.
- James Jacobs (October 2007). “The Demonomicon of Iggwilv: Graz'zt, the Dark Prince”. In Kim Mohan ed. Dragon #360 (Wizards of the Coast), pp. 13–14. Archived from the original on 2009-06-03. Retrieved on 2019-08-27.
- Dale Donovan (May 1998). For Duty & Deity. (Wizards of the Coast), p. 3. ISBN 0-7869-1234-0.