THETIS, daughter of Nereus, an old sea god, was Achilles's mother. Since it was foretold that she would bear a son greater than his father, Jupiter and Poseidon decided to marry her to a mortal (Met IX.221-228, 256-265).
Thetis and the sea gods rescue Demophon from drowning and cast him up on the beach in Thrace, LGW 2421-2426. Chaucer's source for this line is apparently Aeneid V.823. [Achille: Demophon: Phillis]
Thetis occurs once in medial position, LGW 2422.