Echidna

Daughter of Phorcys and Ceto (or in some accounts, of Gaia and Tartarus), Echidna dwelt in a deep cave beneath the earth and was granted immortality. She and Typhon together produced Cerberus, the Lernaean Hydra, the Chimera, the Sphinx, the Nemean Lion, the Colchian Dragon, Scylla, and Ladon.

The upper half of a strikingly beautiful woman with dark eyes; the lower half a monstrous speckled serpent of enormous length. She dwelt in a cave, emerging to devour passing mortals who wandered too close.

Immortal and ageless. Possessed the power to spawn entirely new monster species. Could overwhelm most heroes with serpentine strength. Her cave was a liminal point between the mortal world and the underworld.