The Greek poet Hesiod established that Thanatos was a son of Nyx (Night) and Erebos (Darkness) and twin of Hypnos (Sleep).

"And there the children of dark Night have their dwellings, Sleep and Death, awful gods. The glowing Sun never looks upon them with his beams, neither as he goes up into heaven, nor as he comes down from heaven. And the former of them roams peacefully over the earth and the sea's broad back and is kindly to men; but the other has a heart of iron, and his spirit within him is pitiless as bronze: whomsoever of men he has once seized he holds fast: and he is hateful even to the deathless gods." (Hesiod, Theogony 758 ff, trans. Evelyn-White, Greek epic C8th or C7th B.C.)

Homer also confirmed Hypnos and Thanatos as twin brothers in his epic poem, the Iliad, where they were charged by Zeus via Apollo with the swift delivery of the slain hero Sarpedon to his homeland of Lykia.

"Then [Apollon] gave him [Sarpedon] into the charge of swift messengers to carry him, of Hypnos and Thanatos, who are twin brothers, and these two presently laid him down within the rich countryside of broad Lykia." (Homer, Iliad 16. 681 ff)

Counted among Thanatos' siblings were other negative personifications such as Geras (Old Age), Oizys (Suffering), Moros (Doom), Apate (Deception), Momos (Blame), Eris (Strife), Nemesis (Retribution) and even the Stygian Boatman Charon. He was loosely associated with the three Moirai (for Hesiod, also daughters of Night), particularly Atropos, who was a goddess of death in her own right. He is also occasionally specified as being exclusive to peaceful death, while the bloodthirsty Keres embodied violent death. His duties as a Guide of the Dead were sometimes superseded by Hermes Psychopompos. Conversely, Thanatos may have originated as a mere aspect of Hermes before later becoming distinct from him.

Thanatos was thought of as merciless and indiscriminate, hated by - and hateful towards - mortals and the deathless gods. But in myths which feature him, Thanatos could occasionally be outwitted, a feat that the sly King Sisyphus twice accomplished. When it came time for Sisyphus to die, he cheated Death by tricking him into his own shackles, thereby prohibiting the demise of any mortal while Thanatos was so enchained. Eventually Ares, the bloodthirsty God of War, grew frustrated with the battles he incited, since neither side suffered any casualties. He released Thanatos and handed his captor over to the God, though Sisyphus would evade Death a second time by convincing Persephone to allow him to return to his wife.

"King Sisyphos, son of Aiolos, wisest of men, supposed that he was master of Thanatos; but despite his cunning he crossed eddying Akheron twice at at fate's command." (Alcaeus, Fragment 38a, trans. Campbell)

Thanatos is usually an inexorable fate for mortals, but he was only once successfully overpowered, by the legendary hero and demigod Herakles. Thanatos was consigned to take the soul of Alkestis, who had offered her life in exchange for the continued life of her husband, King Admetos of Pherai. Herakles was an honored guest in the House of Admetos at the time, and he offered to repay the king's hospitality by contending with Death itself for Alkestis' life. When Thanatos ascended from Hades to claim Alkestis, Herakles sprung upon the god and overpowered him, winning the right to have Alcestis revived. Thanatos fled, cheated of his quarry.

Thanatos : Much talk. Talking will win you nothing. All the same, the woman goes with me to Hades' house. I go to take her now, and dedicate her with my sword, for all whose hair is cut in consecration by this blade's edge are devoted to the gods below. (Euripides, Alcestis 19 ff, trans. Vellacott, Greek tragedy C5th B.C.)