The ancient marine known to history as Aetherius was a Blood Angels Terminator Sergeant during and after the Great Crusade and the Horus Heresy. Born on the homeworld of Humanity, Aetherius grew up in the time before the discovery of his Primarch, Sanguinius. At a young age, he was taken to undergo training to be transformed into a Space Marine, using leftover geneseed from the disappearance of the Primarchs, where he was implanted with the geneseed of Legion IX. Once Sanguinius was found on the heavily irradiated moon of Baal Secundus, he, along with the ten thousand marines of the legion were presented to him, where they were rechristened Blood Angels. He would go on to fight through the rest of the Great Crusade by his Primarch's side with an unfaltering loyalty that persisted even after the start of the dreaded Horus Heresy.
After Sanguinius' death at the hands of the Arch-Betrayer Horus, Aetherius, along with the rest of the legion, mourned for their lost father. Afterwards, Aetherius stayed with the Blood Angels after the Legions were broken down into chapters by the Primarch of the Ultramarines, Roboute Guilliman, where he was moved to the chapter's 1st Company, where he served as a Veteran Terminator Sergeant. During a particularly brutal campaign against a small Ork Empire, Aetherius was lost in the palace of the Ork king, and was presumed dead. Thousands of years later in mid-M40, during the Twenty Fifth Founding, a new chapter was formed using the geneseed of the Blood Angels. The new Chapter Master,Terzo Pesaresi, himself a former Blood Angels Veteran Sergeant, spent days looking through the history of the Blood Angels on his way to the new chapter's homeworld, trying to find an apt name for it. Once he stumbled across the tales of Aetherius, he was captivated, and later named his chapter, the Aetherian Warriors, after him.