The Keres are Greek spirits parented by Nyx and Erebos–they are thus the sisters of the Fates. In some stories, they are what was released when Pandora opened the infamous box.
In other stories, the Keres are closely associated with violent deaths, especially murders and combat deaths. They are vulture-like, eating the carrion on the battlefields after ripping the souls of men away from their bodies. They were often depicted with long talons, sharp teeth, and large wings.
The Keres were closely associated with both the Underworld itself and with Hadies, the lord of the Underworld. They’re also called the Death Fates, as they were present as a person’s fate was about to be decided–they weighed the mortal’s life to decide where (s)he ended up after death.
They didn’t have absolute power, though—that was of course reserved for the gods, who could speed up or possibly halt their missions. In some battles, the gods were said to stand by their favorites, their champions, fending of the Keres.
The Keres are curious—sometimes it seems to be a proper name, as though there’s only one. In other stories, there are many, enough of them to consume a battlefield of dead and wounded soldiers. They are always portrayed as vicious, awful and monstrous, the antithesis of their brother Thanatos, the gentler harbinger of death.
*Image via greekmythology.com