Adimarchus

Adimarchus, called the Demon Prince of Madness, was a demon lord and the former ruler of the abyssal layer of Occipitus.

Description
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.

Powers
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.

History
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 and his numerous other schemes, was a component of Graz'zt's plot to unify all demons of the Abyss.