Echidna

Hesiod describes Echidna as a creature of two halves: a bright-eyed, fair-cheeked woman above, and below a vast serpent, speckled and terrible, feeding on raw flesh in a cave under the earth. She does not age and she does not die.

Her importance is genealogical. With Typhon, the most dangerous being ever born, she produced most of the monster population of Greek myth: Cerberus, the Hydra, the Chimera, Orthrus the two-headed hound, and depending on the source the Sphinx, the Nemean Lion and the eagle that tore at Prometheus. Read the labours of Heracles as a family tree and they are mostly one household.

This is why the myths keep sending heroes to the same well. A monster is rarely a random obstacle; it is somebody's child, with sisters and brothers who are also out there. When Heracles kills the Nemean Lion and the Hydra in successive labours, he is working through siblings.

Echidna herself mostly survives the stories about her children. In one account she is finally killed in her sleep by Argus, the hundred-eyed giant — which is a quiet end for the mother of everything that ever ate a hero.

Her cave is described as being under a hollow rock, far from gods and mortals both, which is its own kind of statement. Echidna is not part of the divine order and not opposed to it; she is simply outside, producing the creatures that the order will spend its heroic age putting down. The myths never send anyone to kill her on purpose.