In Greek mythology, Pyrrhus is the grandson of what immortal nereid?
Pyrrhus (Royer):  The libretto, by Fermelhuis, is based on the Greek myth of Pyrrhus, son of Achilles. Achilles:  His mother was the immortal nereid Thetis, and his father, the mortal Peleus, was the king of the Myrmidons.
Thetis