Gifts
by Ursula K. Le Guin
The novel begins with Emmon, a lowland runaway, coming to Caspromant when Orrec and Gry are 16. The children tell Emmon of their gifts, though he is somewhat disbelieving of them.
When he is 13, Orrec seemingly becomes able to use his power, striking an adder dead when it was about to bite his father.
The Caspros visit Drummant, but Ogge is rude to them, and the granddaughter Ogge proposed to betroth to Orrec is found to be mentally disabled.