Abraxus serves as the "Sage Of Secrets" to the Master of Paris, Colette Voltaire. He was formerly an executive assistant to Simon Voltaire, the previous Master of Paris and Colette's father. We first meet him as he is bringing Simon a message from the Incorruptible Library about the return of the Storm King, a message that suggests they've sent earlier messages. Thanks to the machinations of Drusus Beausoleil, Simon's turncoat subordinate, those earlier messages never reached the old Master.
This revelation prompts ✣ the Master to re-open his command chamber (something he hasn't done in recent memory), "suit up", and assume direct control of his city for what proves to be the last time, all with Abraxus as a rather awed witness.
Later on, during a post-battle audience conducted by Colette ✣ for those who aided her, primarily Agatha and Martellus and their respective groups, Ognian sort of technically battles ✣ Abraxus, at least enough to officially claim ✣ the man's rather spiffy hat (see the image in the info box) as a war-prize.
Much later, after Colette has left her city temporarily to aid in successfully defending Mechanicsburg from an attack by the Polar Ice Lords, a highly agitated and somewhat disheveled Abraxas unexpectedly reappears ✣ to report the shocking news that Paris has fallen to Princess Terebithia von Blitzengaard. (It is worth noting that he has replaced his hat with a lookalike or somehow retrieved it.)