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Alexios (Greek: Αλεξιος; 451 BCE – 422 BCE), also known as Deimos, was a Sage of the Cult of Kosmos, an organization active in ancient Greece. Alongside his older half-sister, the famed misthios Kassandra, he was a grandchild of King Leonidas I of Sparta through his mother, Myrrine, as well as a hybrid. His unique heritage made him the target of the Cult as an infant and a false oracle delivered by the Cult deceived Sparta into believing he had to be thrown off Mount Taygetos to his death. Although he survived the fall, Alexios was abducted by the Cultist Chrysis and forced to endure a cruel and torturous childhood. His harsh upbringing warped his mind, brainwashing him into becoming a weapon of the Cult convinced that he was their champion, Deimos. In reality, the other Cultists only saw him as a deranged beast which served their purposes. Not content with their control over Alexios, the Cult sought the assassinations of the rest of his family: Nikolaos, Myrinne, and Kassandra. At a conclave assembled by Deimos, he met his long lost sister for the first time in years while she was masquerading as one of the Cultists in order to learn the whereabouts of their mother. Over the next several years, as the Peloponnesian War raged, Alexios continued serving as Cult's brutal instrument all the while his sister assassinated its members in a desperate attempt to protect her family and redeem him. Her efforts ultimately fell short when Alexios, guilt-ridden over his monstrous actions, forced her to kill him during a final confrontation atop Mount Taygetos.

Alexios

422 BCE
for 422 BCE in The Assassins and Templars Throughout History
Suggested by benpopplewell

Due to the secretive nature of the Assassin-Templar War, there are no definitive in-universe examples of the Assassins or Templars being born and dying in places completely "unrecorded" by human history. With both sides operating covertly within known locations and time periods. However, there are several instances of the Assassins and Templars whose birth and death locations are unknown or hidden sources themselves, making them effectively unrecorded by the historical record. The secret war between the Assassins and Templars, which largely takes place behind the scenes of major world events. Animus simulations allows anyone to explore these hidden histories according to Wiki page as the battles, and secret societies did not appear in standard history books, as long as Abstergo was concerned.