Thanatos was the daemonic god of death in Ancient Greek mythology; daemonic here is used with its classical meaning, which refers to benevolent or benign nature spirits. He did not play a major part in Greek mythology and rarely appeared in any stories, as he was mostly displaced by Hades, the god of the Underworld.
He was the son of Nyx (the Night) and Erebus (the Darkness), while his twin brother was Hypnos (the Sleep). His nephew was the god of dreams himself, Morpheus. It is said that Thanatos offered Morpheus his daemonic wings.
Thanatos was believed to be merciless and indiscriminate, because of this, both mortals and gods hated him. However, he could sometimes be outsmarted. In a myth, Thanatos was told by Zeus to chain King Sisyphus in the Underworld, as it was time for him to die.
Sisyphus managed to chain Thanatos in his own fetters, thus protecting all mortals from dying while the god was chained. In the end, the god of war Ares, angry that at the wars he waged had no deaths, freed Thanatos and gave Sisyphus to him.