Thanatos was the mortal god of the death. He was the twin brother of Hypnos, and main servant of Hades. When he dies in anime, his star in his head disappears.
Thanatos is imprudent, and impulsive, but the another main feature of Thanatos, is his position about "death penalty"; according he, all his enemies have be killed, with out forgive and no think twice and don't give importance to the consequences. Because this, is is impulsive to kill directly his enemy. He is a enemy of Poseidon and Athena.
According Lost Canvas, Thanatos with Hypnos in the Holy War of XVI Century, did Athena dies after Hades defeat. So, Sage and Hakurei gets worried with them menace.
In the Century XVIII holy war, Sage goes to arrest Thanatos in a box, and after Manigold sacrifice (threw himself and the human body of Thanatos in a dimension that destroy all non-divin things), Sage put Thanatos souls in the box.
In Lost Canvas, he was a great friend of Pandora.
In the Saint Seiya classic series, Thanatos and Hypnos was freed by Pandora and he and Hypnos promises to Pandora, eternal life if she cares of the Hades soul and Hades host, being his elder sister. Pandora was brainwashed and only saw the wolrd around she in gray and black colors, thought Hades will purifies this world
But, when Pandora betrays Hades, Thanatos, with his wickedness, kill Pandora, disregarding all what she did to Hades.
Seiya arrives in the Elysium, and meet Thanatos, that tries kill Seiya; but, when Seiya hurts Thanatos, he decides kill Seika, but Shaina, Kiki, Marin, Ban, Ichi, Geki, Jabu and Nachi saves she, making a shield with his bodys. Shiryu, Hyoga, Shun and Ikki arrives to help in the battle, vainly. But, Poseidon sends gold clothes to Seiya and the other uses, however, Thanatos destroy these cloths (this never happened).
Only when Seiya gets the Divin Cloth, Thanatos was defeated and killed in this time.