Proteus was a Scout Novice originally from Captain Leonatos' one-hundred company. When Leonatos lost the sacred Sword of Belarius during the Last Siege of Sekundar Prime, he was exiled along with other 9 survivors from the original company and Proteus joined his commander in a Bloodquest to retrieve the artifact. Proteus was noted by his bravery and abilities beyond question, but always had was headstrong and impatient nature, untempered by experience. He and Lysander trained together on Baal and were close as brothers.
Their search for the Blade Encarmine landed them to the Daemon World of Eidolon, a planet twisted and corrupted by the power of Chaos and the Warp after being trapped inside the Eye of Terror. Eidolon's control was contested between all four Gods of Chaos, who fought for supremacy through their champions. When the group crossed the realm of Slaanesh, Proteus lost his way having been seduced by a Slaaneshi princess who corrupted him. Having pledged his soul to Chaos, Proteus' body was mutated and twisted with the pleasures and sensations given by his mistress and he confronted his brother Lysander while the latter was lost in Tzeentch's domain. He survived the encounter and remained behind in the Daemon World as his brothers returned the Blade Encarmine to their chapter.
Proteus would later serve under the Tzeentchi Daemon Prince that possessed Captain Leonatos, while still worshiping Slaanesh. After Lysander and Cloten returned to Eidolon, he was able to spot them while they were masquerading as renegades and had them captured. At first, he wanted to make them his playthings for him and his harem-girls, but Leonatos had different ideas: he had the two fight each other to the death and the survivor would have been subjected to a horrible, grisly torture and Lysander came out victorious. Proteus personally oversaw his comrade being continuously tortured by his pain technician and offered one last change to turn to Chaos, but Lysander refused. While contemplating in his chamber with his harem, he was attacked by Cloten who had survived the fight and released Lysander, who promptly beheaded his former brother
Powers & Abilities
As an Adeptus Astartes, Proteus is gifted with superhuman strength, speed, endurance, reflexes, has a mighty power armor, does not need to sleep and eat. Due to being a Blood Angel, he is susceptible to falling to the Red Thirst - a vampiric condition that compels its victims to drink blood and subsequently the Black Rage, an incurable state of eternal fury.
After falling to Chaos, Proteus became immortal and immune to aging. As an Slaanesh worshiper, he has being given several mutations to increase sensations and pleasure.