Thanatos

Thanatos (死を司る神タナトス, Shi o tsukasadoru kami Tanatosu?, with shi o tsukasadoru kami meaning "death-ruling god") is a Greek god and one of the antagonist deities in the universe of the Manga Saint Seiya, written by Masami Kurumada and later adapted into an anime. Kurumada created him based on the mythological figure of the same name.
As in Greek mythology, he personifies mortality and death and is the twin brother of Hypnos, the Greek god of sleep. In Kurumada's mythos, although he serves as a subordinate to Hades, the Emperor of Darkness and ruler of the Underworld, he possesses all divine attributes. As Hades's lord, he is a sworn enemy of Athena, the protector goddess of Earth.
Kurumada introduced Thanatos in the final parts of the Hades arc of his manga, specifically in volume 27. He is portrayed as very cruel, merciless, hateful toward humans, haughty, and unhesitating in whimsically inflicting death. Athena sealed him along with his brother after defeating them in her last battle against Hades in the 18th century. Thanatos spent 243 years sealed in a box marked with the Seal of Athena. He regained his freedom only after a curiosity-driven 3-year-old Pandora opened the box. To her, he announced that Hades's resurrection was near and that she would serve them henceforth.
As described by the Pegasus Saint Seiya upon their first encounter, he is identical to his brother Hypnos, with the only differences being Thanatos's silver-colored eyes and hair, compared to Hypnos's golden ones. A strange-looking star is placed on his forehead. Unlike his brother Hypnos, who is more calm and calculating, Thanatos is prone to losing his temper and acting without regard for consequences when influenced by rage.
His dwelling place is Elysion, where he spends most of his time accompanied by scores of Nymphs and his twin brother Hypnos, waiting to fulfill their lord Hades's bidding. Thanatos offered fierce opposition to the Saints of Athena when they tried to rescue their goddess from certain death and stop Hades's Greatest Eclipse, which was meant to eradicate mankind.
Thanatos's duty is to impart death on all living beings when their time has come, or as divine retribution. Before his formal introduction in the anime adaptation, he appeared in the opening of the Inferno stage of the Hades arc episodes (episode 128 and on), standing next to his brother Hypnos and overshadowed by Hades. His full appearance in the anime adaptation occurred in 2008, in the final episodes adapting the last two volumes of the manga, in which his participation is contained (volumes 27 and 28), 16 years after Kurumada introduced him in the manga. From episode 142 onward, his silhouette in the anime opening revealed his features fully.