Source: Ruins of the Radiant Siege, pg(s). 25
Kharnas was born to a mortal woman and a planetar angel in Calistria's service named Avsheros the Delightful. Asvheros' dalliance and his son's birth were a political misstep, and was one of the factors that led to the angel's fall from grace. Avsheros blamed his lover and child for his downfall, and gave them away as gifts to Hamza bin Gamal, an efreeti wizard living in the City of Brass. Kharnas was able to learn rudiments of magic from his master's arcane library, and enjoyed a well-provided but loveless position of servitude even as his mother was sold to a slave trader. These trials early in life led Kharnas to become an angry and resentful child.
Hamza eventually grew tired of Kharnas and sold him to night hags of the Coven of the Harvesters of the Cage. The hags recognized Kharnas' magical skill and celestial heritage and chose not to simply consign him to the soul trade. They instead sold him to Ceolaeros the Twice-Risen, a former Horseman of Pestilence who uniquely abdicated his throne instead of being destroyed by a rival. Kharnas served Ceolaeros for many years until he eventually set off on his own, although his soul had become warped and stained by his association with the archfiend.
After leaving Ceolaeros' service, Kharnas sought out the being he believed had engineered both Ceolaeros' rise to and fall from power and his own entrance in the fiend's ownership—Tegresin, the Laughing Fiend. Once he reached the Prison of the Laughing Fiend, Kharnas struck a bargain with the fiend while fully aware of Tegresin's nature as a weaver of lies. Kharnas bargained for a means to avenge himself on his father; smiling, Tegresin gave him the Radiant Spark, a device capable of enslaving planar beings.
Kharnas quickly took vengeance of his father, and afterwards contented himself to a lifetime of tormenting angels for eternity. However, in 1619 AR, a century after the bargain was struck, Tegresin demanded his payment for the bargain. Kharnas was charged with seizing the Starstone for the Laughing Fiend. Kharnas launched an attack on Absalom, intending to physically take the artifact and deliver it to Tegresin. He came closer to seizing the city than many other besiegers did, but Absalom's defenders were, however, able to defeat him. When Kharnas' death was imminent, Tegresin claimed him as payment, but deliberately left the Radiant Spark behind.