Lord Proust came into being as one of Fabius Bile's experiments. He and 20 others that did not die in Bile's experiments where given to Warmaster Abaddon. He was transported to the Gothic Sector and was part of the carage at Corilia. He quickly gained favour with both Adaddon and the Dark Gods. Proust would go on to fight on Cadia and Medusa V, where he as a Chaos Lieutenant sent thousands of his soldiers to be cut down in bloody battles that lasted for several weeks. For his actions he became a Chaos Lord and was ordered by the Warmaster to take a warband to Agrippa and enforce his will.
Lord Proust is dedicated to his own superiority, Proust seeks to guide his warband to the ultimate conquest of Agrippa. As far as he is concered, humans are unfit to rule. Proust is an ambitious and daring commander. Programmed with small-unit tactics in mind, Proust occasionally has difficulty coordinating large numbers of less disciplined troops, but is brilliant on the battlefield.
Proust's programming drives him to lead at all times. He is not satisfied unless he is driving his followers toward an objective, each loftier than the last. In Proust's case this drive to lead also manifests itself as severe megalomania. He is absolutely positive that no other person (not even Abaddon) is better suited to command than he is.
Proust's main enemy is his own megalomania. His conviction that he is invincible sometimes overrides his tactical judgment, causing him to take unwise risks. Lady Ganes, a Slannesh Follower has become his lover and anchor in many ways.
Proust main concern is enforcing the will of the Warmaster to the forces of Chaos already in Agrippa. And gaining a base so that he can make his bid of conquest.