J.R.R. Tolkien> The Silmarillion

"...but Celegorm and Curufin vowed openly to slay Thingol and destroy his people, if they came victorious from war, and the jewel were not surrendered of free will."

-J.R.R. Tolkien
The Silmarillion

Celegorm was the third of the sons of Fëanor. He swore the Oath of Fëanor along with his brothers and followed it with the greatest zeal of any of his siblings. On the Noldor's return to Middle-Earth, Celegorm was instrumental the battle of Dagor-nuin-Giliath and in guarding Himlad with his brother Curufin. After their defeat in the Dagor Bragollach, he fled to Nargothrond.

Along with his brother, he opposed Finrod's wish to aid Beren and Lúthien in their quest for the Silmarils. Here he renewed his vow. Afterwards, Celegorm met Lúthien and fell in love with her. Celegorm and Curufin were then banished from Nargothrond when Lúthien's father Thingol found out of Celegorm's plan to marry her.

He was slain at Menegroth by Dior when when he and his brothers tried to recover the Silmarils kept there. He was known as Celegorm the Fair because of his complexion. He was also the proudest of his brothers and acted more by brute force then by deceit and treachery.