Rathma, whose birth name was Linarian, is a first generation Nephalem and the progenitor of the Priests of Rathma, also known as the Necromancers. He is the son of Lilith and Inarius. He is not on good terms with his parents, as Lilith destroyed the rebel angels and demons only to use the Nephalem's power for her own ends, while Inarius believed that the Nephalem should worship him alone.
Rathma and the other members of the first generation, such as Bul'Kathos, retained their supernatural powers, although all later generations needed to be trained or discover magic some other way. Many of the firstborn were destroyed by Inarius, who would not tolerate defiance. This prompted Rathma to feign death and flee to a mysterious nether-realm; the realm of Trag'Oul, a dragon-like being who acts as Sanctuary's guardian.
Rathma became the first disciple of Trag'Oul, who taught him to preserve what Trag'Oul called the Balance. Their goal was to keep the balance between the Burning Hells and the High Heavens and prevent either from interfering too much in Sanctuary's affairs.
Rathma selected Mendeln as one of his earliest disciples. Mendeln and his brother Uldyssian eventually met Rathma and Trag'Oul. Uldyssian, eager for vengeance against Lilith and disliking Inarius, joined Rathma on his journey to the Worldstone to change its inner workings and to free the power of the Nephalem.
Rathma was horrified to learn that the High Heavens had discovered Sanctuary. In his mind, the angels are no better than the demons, as the angels are likely to obliterate the entire planet simply for having demons present.
Rathma was briefly banished by his father to a vast realm of 'nothingness'. This was also the same realm where Inarius had previously banished his former love Lilith to. Only by the combined efforts of Trag'Oul and Mendeln could Rathma be rescued.
Rathma later assisted during the final Battle of the Golden Path, along with the remaining firstborn. After the battle was won, Rathma explained to Mendeln that his time on the world would eventually come to an end, instructing him to train additional disciples so that the Balance would always have dedicated guardians.