Adimarchus; also known as the "Demon-Prince of Madness", is a fallen Celestial who is completely insane.

Adimarchus was a fallen celestial. It is not known how or why he fell from grace, but what is known is that he eventually led an army of demons to invade the upper plane of Celestia. The angels of Celestia repelled the invasion by casting part of Celestia (along with Adimarchus and the invading demonic horde) into the Abyss, specifically, the 507th layer known as Occipitus.

Despite his defeat, Adimarchus incorporated the wreckage of Celestia into the layer, and bydoing so, he became both the ruler of Occipitus, and a powerful demon lord in the Abyss. Eventually, the Demon-Lord of Occipitus came into conflict with another Demon-Lord named Graz'zt. Shortly afterwards, Adimarchus disappeared.

It turned out that an aasimar paladin named Athux went to Occipitus on a quest to redeem Adimarchus's soul. Athux fought his way through the layer and into the throne room of the fallen celestial/ demon prince. The two fought practically to a standstill until Adimarchus gained the upper hand and sundered Athux's weapon. Adimarchus would have killed Athux but in a rare moment of compassion sparked by his celestial nature could not bear to do it. Instead, Adimarchus kept Athux prisoner and attempted to seduce him to the side of Evil. Athux, on the other hand, fueled in Adimarchus a hatred against demonkind. A friendship actually bloomed between the two former arch-foes, thus causing some of Adimarchus's minions to turn against him.

When Adimarchus planned an assault on Graz'zt, Graz'zt himself conspired with Adimarchus's traitorous lieutenants. In the ensuing battle between the two hordes, Graz'zt captured Athux and imprisoned him in Carceri. Adimarchus abandoned his layer and his armies and went to Carceri to free Athux and, perhaps, finally seek redemption. He found Athux, stricken with insanity in an asylum called Skullrot, run by one of Graz'zt's minions, a lich-fiend named Dark Myrakul.

Dark Myrakul offered Athux's freedom if Adimarchus himself would take his place in the asylum. Adimarchus readily agreed, and when Athux was freed, the truth came out.

Athux was pretending to be insane. In fact, he was pretending to be an aasimar paladin. Athux turned out to be a son of Graz'zt who had been manipulating Adimarchus from the very beginning. When Adimarchus realized the truth, his mind snapped. Adimarchus has been imprisoned in Skullrot ever since, and as the years went by, his hatred and insanity swelled until he transformed into two beings; one angelic, and one demonic

Adimarchus has two forms. The first may be what he looked like when he was a Celestial, that of a pale purple-skinned hairless humanoid with razor sharp metallic wings. Golden tattoo-like patterns shift across his chest. His other form is that of a lithe humanoid with ash-black skin and four tentacles protruding from his back that end in lamprey-like maws. Adimarchus can switch between these two forms at will. To destroy Adimarchus is like fighting two separate creatures; killing just one form but not the other does not slay him. Damage sustained in one form is not carried over to the other. As a result of his celestial origins, any spells or spell-like abilities with the good descriptor treats Adimarchus as if his alignment were good.