Thanatos was born in the earliest generation of the cosmos, when the world was still dark and the first divine beings were only just emerging from Chaos. His mother was Nyx, the Night, one of the first children of Chaos, a goddess so powerful that even Zeus feared to offend her. His father was Erebus, the Darkness, the deep shadow that filled the spaces between earth and sky before the sun ever rose. Together they produced a brood of personified spirits, each one representing an aspect of the human condition that mortals could not escape. Hypnos, Sleep, was Thanatos's twin, born at the same moment, and the two were inseparable in Greek thought and art. Where one went, the other was never far behind, for sleep is the brother of death, and every night the living taste a little of what the dead know forever. Their siblings were no less formidable: Nemesis, the spirit of righteous retribution; Eris, the spirit of strife; the Keres, the spirits of violent death who haunted battlefields like vultures; and the Moirai, the three Fates who spun, measured, and cut the thread of every human life. Thanatos was not the worst of this brood, nor the most terrible. He was simply the most inevitable.

In the Greek imagination, Thanatos was not a monster. He did not cause suffering, and he did not delight in pain. He was the end of suffering, the final release from the burdens of the body. He was depicted as a beautiful young man, winged like an angel, with a gentle face and a sword that remained sheathed unless he was forced to use it. His torch was inverted, burning downward rather than upward, a symbol that his light led not to life but to the darkness beyond it. When he came for the dying, he did not drag them screaming. He lifted them softly and carried them on his wings to the house of Hades, the god who ruled the underworld, where the souls of the dead received their final judgement. The Greeks did not fear him as they feared the Keres, who tore at the bodies of the slain, or the Furies, who drove murderers mad with guilt. They feared him only because he could not be refused.

His most famous appearance in the heroic myths came during the Trojan War, the ten-year siege of the city of Troy that pitted the Greeks against the Trojans in the most devastating conflict of the legendary age. Sarpedon was the king of Lycia, a kingdom in southern Asia Minor, and the son of Zeus and the mortal woman Laodamia. He fought on the side of the Trojans, and in the sixteenth book of the Iliad, the epic poem of Homer, he met his death at the hands of Patroclus, the beloved companion of Achilles, the greatest Greek warrior. When Sarpedon fell, Zeus wept, for the king was his own son, but he did not intervene to save him. Instead, he commanded Hypnos and Thanatos to carry Sarpedon's body from the battlefield back to his homeland in Lycia, where he could be given a proper funeral and buried with honour among his own people. The two winged brothers descended upon the corpse, lifted it gently from the blood-soaked ground, and flew with it across the sea and the mountains to Lycia. The scene is one of the most beautiful in the Iliad: death not as a horror but as a service, the gentle brother lifting the fallen king while sleep guards his body from desecration. In vase painting, the two brothers are shown as identical winged youths, one carrying the body and the other walking beside it, their faces calm, their movements deliberate, performing the last kindness that a hero could receive.

But Thanatos was not invincible, and his most humiliating defeat came at the hands of Sisyphus, the king of Corinth whose cunning was legendary. Sisyphus had betrayed a secret of Zeus to the river-god Asopus, and in punishment Zeus sent Thanatos to fetch the king to the underworld. Sisyphus, however, was ready. When Thanatos arrived with his chains, the chains that bound the souls of the dying and prevented their escape, Sisyphus asked the spirit of death to demonstrate how they worked. Thanatos, proud of his craft and perhaps unsuspecting, fastened the chains around his own wrists to show their mechanism. Sisyphus tightened them, and Thanatos was trapped. For a time, no mortal on earth could die. The old grew older without end, the wounded could not find release, and sacrifices made to the gods produced no smoke, for the fire could not consume what death could not claim. The gods themselves were inconvenienced, for the natural order had been disrupted. Finally Ares, the god of war, was sent to free Thanatos, for a world without death made war meaningless, and Sisyphus was dragged to the underworld. But the king had one more trick, and he escaped again, leaving Thanatos to resume his endless task with the memory of his own imprisonment.

The greatest confrontation between a mortal and Thanatos came in the tragedy of Alcestis, the queen of Pherae in Thessaly. Her husband Admetus, the king of Pherae, had been fated to die young, but he had won the favour of Apollo, the god of prophecy and music, who had convinced the Moirai, the three Fates, to allow someone else to die in his place. Alcestis, his wife, volunteered, and when her appointed day came, Thanatos arrived to claim her. He was not cruel about it. He simply did his duty, cutting the thread of her life and preparing to lead her to the underworld. But Heracles, the greatest hero in Greece, happened to be staying in the palace as a guest of Admetus, who had hidden the truth of his wife's sacrifice from his friend out of shame. When Heracles discovered what had happened, he was furious that his host had concealed such grief, and he resolved to set things right. He went to the tomb where Alcestis's body lay and waited for Thanatos to come and claim her spirit. When the winged spirit arrived, Heracles seized him by the throat and wrestled him to the ground. The fight was brutal, man against the personification of death itself, but Heracles was stronger than any mortal and most gods, and he forced Thanatos to release Alcestis. The spirit of death, bound by the hero's grip, had no choice but to let her go, and Alcestis returned to life, pale and silent from her journey to the edge of the underworld, but alive. It was the only time in all Greek myth that a human being defeated death in physical combat, and it proved that while Thanatos was inevitable for ordinary mortals, he was not absolute before the strength of a hero who had completed the labours of the gods.

In later Greek thought, particularly in the philosophy of the classical period, Thanatos became a subject of deep reflection. The philosopher Epicurus argued that death was nothing to fear because where death is, we are not, and where we are, death is not, making the two mutually exclusive. The playwrights who staged his appearances, from Aeschylus to Euripides, used him to explore the boundary between life and death, the moment when the soul departs and the body becomes a shell. In Roman times, he was identified with Mors or Letum, though the Romans had less to say about him than the Greeks, perhaps because their imagination of death was less personified and more abstract. But in the Greek world, Thanatos remained a presence, a winged figure in the corner of the sickroom, a shadow at the edge of the battlefield, the gentle brother who comes for everyone in the end.