It is fairly well known that Medusa, the snake-headed Gorgon, had two sisters, Stheno and Euryale.
While the story of Medusa is concluded when Perseus beheads her, in modern retellings and versions, we don't hear about her sisters. What happens to the other two Gorgons? Are they immortal? Do they seek revenge?
In pseudo-Apollodorus' version, Medusa's sisters sought revenge on Perseus, who escaped them by using the Cap of Hades (which rendered its wearer invisible):
So Perseus put the head of Medusa in the wallet (kibisis) and went back again; but the Gorgons started up from their slumber and pursued Perseus: but they could not see him on account of the cap, for he was hidden by it.
Source: Apollod. 2.4.3
Also, at least according to Hesiod, Medusa's sister were immortal:
And again, Ceto bore to Phorcys the fair-cheeked Graiae, sisters grey from their birth: and both deathless gods and men who walk on earth call them Graiae, Pemphredo well-clad, and saffron-robed Enyo, and the Gorgons who dwell beyond glorious Ocean in the frontier land towards Night where are the clear-voiced Hesperides, Sthenno, and Euryale, and Medusa who suffered a woeful fate: she was mortal, but the two were undying and grew not old.
Source: Hes. Th. 275
