Leonatos belongs to the Blood Angels Legion of Adeptus Astartes, the superhuman soldiers of the Imperium of Men and foremost defenders of humanity. The Blood Angels are noted for their savage nature in combat, being extremely long-lived even among the Space Marines and resembling mythological vampires, due to being afflicted with literal urge to drink blood. Leonatos and his one-hundred company of men were sent to defend a important Warp beacon from a Ork invasion and to aid in his quest, he was given the Blade Encarmine - a Chapter relic sacred to the Blood Angels. During the fight, the artifact was lost and Leonatos was put on trial for losing the blade. Found guilty, he was exiled and until he had retrieved the Blade Encarmine along with the other ten survivors of the original 100 marines.
The search for the blade lead the band into the Eye of Terror, on the Daemon World of Eidolon, a planet divided into four separate empires, each claimed by one of the Chaos Gods. In the end, only three of the original band remained before recovering the Blade. Leonatos himself was lost to the malefic powers of the Warp as he fought one last stand against an army of daemons to buy enough time for Sargeant Cloten and Scout Novice Lysander to return to the material plane.
Unknown to them, Leonatos was not killed but possessed by a Daemon who had previously tormented along the way in Eidolon and was supposedly slain by him before the survivors escaped. Using Leonatos' tactical knowledge and imbuing his body with arcane powers, the Daemon conquered the rest of Eidolon, formed an warfleet and manipulated an Ork to attack an imperial armada, leaving the system unprotected for his own attack. This was revealed when Cloten and Lysander returned to Eidolon to search for Captain Leonatos only to find that the men they once knew was no more, replaced by a perfidious Daemon of the Warp.
Leonatos forced his former brothers to fight to the death in the arena for his own amusement against other Daemonic beings and between each other. Whomever loses, is given a swift death while the winner will be horribly tortured and tormented until they turn themselves to Chaos. Lysander apparently kills Cloten and is subjected to an visceral torture by Leonatos' servants, though he preferred death than become another Chaos plaything. Unexpectedly while in his own private quarters, Leonatos briefly returned to control himself from the Daemon's possession after seeing his brothers again. He orders his men to divert their original plan to foil the Daemon's crusade before it returned to take control of his body.
Unbeknownst to him, Cloten had managed to survive the duel with Lysander, fought his way through the vessel and released his brother from the torture chamber. The two marines raised up a rebellion among the Chaos slaves in order to bring down Leonatos' warship. Lysander confronted his former captain and after a grueling battle, Leonatos was given a lethal blow that allowed him wrestle control of his body from the Daemon once again. He reveals he settled the ship to course with the Imperial fleet, setting the Chaos armada in to a trap dooming everyone. He instructs Lysander to abandon the vessel through a escape pod before its destroyed by the Imperials. Finally free from the Daemon's influence, Leonatos finally dies as himself going down with his warship.