Who was the Greek primordial god of death?

The Greek primordial god of death was Thanatos. More precisely, Thanatos was the personification of death, especially the inevitable and often peaceful end of mortal life.

Hesiod places Thanatos among the offspring of Nyx, the primordial goddess of night. He was the brother of Hypnos, the personification of sleep, and belonged to a large family of abstract powers that included Moros, doom; Geras, old age; and Oizys, suffering.

Thanatos was not the ruler of the Underworld. That role belonged to Hades, who governed the realm of the dead. Nor was Thanatos identical to the Keres, female spirits associated with violent death. In some traditions, he represented a quieter death, while the Keres embodied deaths on the battlefield.

Myths involving Thanatos often emphasize that death cannot be permanently escaped. Sisyphus once tricked and chained him, temporarily preventing mortals from dying, until Ares freed Thanatos and restored the natural order.