Apollodorus, Library and Epitome 2.4.2
Perseus beheading Medusa with Athena's help
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Polydectes, brother of Dictys, was then king of Seriphus and fell in love with Danae, but could not get access to her, because Perseus was grown to man's estate. So he called together his friends, including Perseus, under the pretext of collecting contributions towards a wedding gift for Hippodamia, daughter of Oenomaus. Now Perseus having declared that he would not stick even at the Gorgon's head, Polydectes required the others to furnish horses, and not getting horses from Perseus ordered him to bring the Gorgon's head. So under the guidance of Hermes and Athena he made his way to the daughters of Phorcus, to wit, Enyo, Pephredo, and Dino; for Phorcus had them by Ceto, and they were sisters of the Gorgons, and old women from their birth. The three had but one eye and one tooth, and these they passed to each other in turn. Perseus got possession of the eye and the tooth, and when they asked them back, he said he would give them up if they would show him the way to the nymphs. Now these nymphs had winged sandals and the kibisis, which they say was a wallet. They had also the cap . When the Phorcides had shown him the way, he gave them back the tooth and the eye, and coming to the nymphs got what he wanted. So he slung the wallet (kibisis) about him, fitted the sandals to his ankles, and put the cap on his head. Wearing it, he saw whom he pleased, but was not seen by others. And having received also from Hermes an adamantine sickle he flew to the ocean and caught the Gorgons asleep. They were Stheno, Euryale, and Medusa. Now Medusa alone was mortal; for that reason Perseus was sent to fetch her head. But the Gorgons had heads twined about with the scales of dragons, and great tusks like swine's, and brazen hands, and golden wings, by which they flew; and they turned to stone such as beheld them. So Perseus stood over them as they slept, and while Athena guided his hand and he looked with averted gaze on a brazen shield, in which he beheld the image of the Gorgon, he beheaded her. When her head was cut off, there sprang from the Gorgon the winged horse Pegasus and Chrysaor, the father of Geryon; these she had by Poseidon.