Thanatos was the Greek personification of death. His twin brother was Hypnos (Sleep) and his mother was Nyx. Thanatos, Hypnos, and Nyx all live at the ends of the earth. Erebus may have been his father.
In Hesiod's Theogony, Thanatos is described as pitiless, making even the gods hate him, even though he cannot kill them since they are immortal. Thanatos appears in Iliad XVI to carry back the body of Sarpedon to Lycia. Thanatos can be overcome, as when Herakles, after wrestling Thanatos, brings Alcestis back from the Underworld. Pherekydes tells of Sisyphos binding Thanatos with the result that no one can die (source: Timothy Gantz).