Monstrous snake woman with a human head and snake-like body. Has a forked tongue, vicious fangs, green scaly skin and the eyes of reptile. . . .
In Greek mythology, Echidna (Greek: Ἔχιδνα, ekhis, ἔχις, meaning "she viper") was half woman half snake, known as the "Mother of All Monsters" because most of the monsters in Greek myth were mothered by her. Hesiod described her as:
[...] the goddess fierce Echidna who is half a nymph with glancing eyes and fair cheeks, and half again a huge snake, great and awful, with speckled skin, eating raw flesh beneath the secret parts of the holy earth. And there she has a cave deep down under a hollow rock far from the deathless gods and mortal men. There, then, did the gods appoint her a glorious house to dwell in: and she keeps guard in Arima beneath the earth, grim Echidna, a nymph who dies not nor grows old all her days.
According to Apollodorus, Echidna was the daughter of Tartarus and Gaia, while according to Hesiod, either Ceto and Phorcys or Chrysaor and the naiad Callirhoe were her parents. Another account says her parents were Peiras and Styx (according to Pausanias, who did not know who Peiras was aside from her father). Echidna was a drakaina, with the face and torso of a beautiful woman (depicted as winged in archaic vase-paintings) and the body of a serpent, sometimes having two serpent's tails. She is also sometimes described, as Karl Kerenyi noted, in archaic vase-painting, with a pair of echidnas performing sacred rites in a vineyard, while on the opposite side of the vessel, goats were attacking the vines: thus chthonic Echidnae are presented as protectors of the vineyard.
The site of her cave Homer calls "Arima, couch of Typhoeus". When she and her mate attacked the Olympians, Zeus beat them back and punished Typhon by sealing him under Mount Etna. However, Zeus allowed Echidna and her children to live as a challenge to future heroes. Although to Hesiod (Theogony above), she was an immortal and ageless nymph, according to Apollodorus, Echidna used to "carry off passers-by", until she was finally killed where she slept by Argus Panoptes, the hundred-eyed giant.
Source(s): Percy Jackson and the Olympians - The Ultimate Guide ;;Wikipedia