Following the Dagor Aglareb, the Glorious Battle, Turgon returned to the secret vale shown to him by Ulmo, accompanied by the best of his people. There in secret, the city of Gondolin is finished after 250 years. Ulmo sets his protections that none should find the city against Turgon’s will, and Turgon’s people effectively disappeared from the map. Ulmo’s warning: Turgon, too, falls under the Doom of Mandos, and so the work of his hands might yet fall to treason within the walls of Gondolin. Ulmo also promised to send a messenger should this betrayal happen. Gondolin thrived, rivaling Tirion in Valinor. And we are told that Turgon’s daughter Idril is the fairest in the land.
Meanwhile, Galadriel and Melian are having some pretty deep conversations over in Doriath. She told some of the history of her people to Melian, but left out the bts about the Oath, the Kinslaying, and the burning of the ships. Melian tells Thingol, and ignoring Melian’s sage advice against the threat of treason (why would you ignore her? She’s a Maia!), Thingol is convinced the Noldor will still aid against the threat of Morgoth.
The Sindar, being rather trusting of the rumors they hear, got wind of the story of the Noldor thanks to Morgoth’s rumor mill. Cirdan took the news to Thingol, who then went to Finrod. Ultimately the people of Finarfin were guiltless in the Kinslaying, but their silence earned Thingol’s anger. Angrod revealed everything, and Thingol’s ruling is the stuff of legend.
He agrees not to shut his borders against them or the people of Fingolfin, for their atonement upon the ice was enough. But he opted for a different kind of genocide to their cultural identity. Thingol banned the Quenya language within his borders. The Sindar refused to speak it. It became a language spoken only by the lords of the Noldor amongst themselves, in much the same way Latin became a dead language of high scholarship after the fall of Rome.
The sons of Finarfin departed with a heavy heart, for the Noldor ever after would dwell in the shadow of Fëanor. When Nargothrond was complete, the sons of Finarfin gathered for a feast. Galadriel came there from Doriath, and she asked Finrod why he had no wife. Until that point, he had no answer, but he realized he loved Amarië of the Vanyar, and “she went not with him into exile.”