Gorgoneion
The Visage of Protection
Homer's Iliad already sets a Gorgon's head on Athena's aegis and on Agamemnon's shield, yet never names Medusa. That Athena set Perseus's trophy in her shield is told in the Library of Apollodorus, of the first or second century AD.
Homer, Iliad 5.738-742 and 11.32-37 (trans. Samuel Butler, Project Gutenberg 2199); Hesiod, Theogony 270-294 (trans. H. G. Evelyn-White), naming Medusa and Perseus; Apollodorus, Library 2.4.3 (trans. J. G. Frazer, Loeb, 1921)
The face of a Gorgon, often Medusa, shown head-on on Greek shields, roof-tiles and cups to ward off harm.
More context
The gorgoneion is the frontal face of a Gorgon, often Medusa, with staring eyes, snakes for hair and a lolling tongue. Homer's Iliad sets it on Athena's aegis and on Agamemnon's shield; later tellers, such as Apollodorus, said Athena placed there the head Perseus had cut off. In Archaic Greek art it appears on temple antefixes, shields, coins and the insides of drinking cups. Most scholars take its purpose to be apotropaic: a monstrous face set to frighten harm away.
Light
Offers fierce psychic protection, the setting of ironclad boundaries, and the courage to look terrifying truths directly in the eye.
Shadow
The risk of becoming the very monster one fights, harbouring paralysing anger, or projecting intense hostility to push others away.