Sargatanas is a noble Demon Major (fallen Seraph), who rules his city-state, as much as is possible in Hell, after the pattern of the Above. In contrast is Beelzebub, the Fly (appointed by Lucifer, who has vanished), official ruler of Hell, who does all in his power to keep it a place of extreme evil and torment. In a moment of illumination, Lord Sargatanas seizes the wild aspiration that a demon might be pardoned and readmitted into Heaven, if only he could prove he has thoroughly renounced Lucifer's rebellion and re-embraced the Light. The siege of Hell's capital, Dis, becomes the path to a forgiveness desperately hoped for.
Which gives us a clue to his origin:
A seraph (Heb. שׂרף, pl. שׂרפים Seraphim, lat. seraph[us], pl. seraphi[m]) is one of a class of celestial beings mentioned once in the Hebrew Bible (Tanakh or Old Testament), in Isaiah. Later Jewish imagery perceived them as having human form, and in that way they passed into the ranks of Christian angels. In the Christian angelic hierarchy, seraphim represent the highest rank of angels.