Oath of Fëanor
The dreadful irrevocable oath taken by Fëanor and his seven sons in the city of Tirion. They swore in the name of Ilúvatar that they would not rest until the three Silmarils were in their hands, and to make war on any who withheld them. Tolkien remarked that it was an oath which should never have been taken, reflecting on the Biblical passage concerning such oaths.
At the time the Oath was sworn, Morgoth held all three of the Silmarils, having stolen them from Fëanor's stronghold at Formenos. Driven by the unbreakable Oath, Fëanor led the greater part of the Noldor out of Aman and back to Middle-earth, in hopeless pursuit of the Dark Lord. Fëanor himself was mortally wounded by Balrogs in the Noldor's first assault. From that time on, while Morgoth held the three Silmarils in his Iron Crown, the Elves of Beleriand were bound by a common enemy.
The true danger of the Oath was revealed after Beren and Lúthien recovered one of the three Silmarils from the depths of Angband. Thereupon, the Sons of Fëanor made war upon the other Elves of Beleriand for the recovery of the Jewel. They attacked and destroyed the kingdom of Doriath, killing Thingol's Heir Dior. When the Silmaril escaped them there, they discovered that it was held at the Mouths of Sirion by Dior's daughter Elwing; again they attacked, and again the Silmaril escaped them. Carried out to sea by the power of Ulmo, Elwing brought it to her husband Eärendil the Mariner, and they sailed with it back into the West.
That Silmaril was lost to Fëanor's sons, but two more remained on Morgoth's Crown. After the War of Wrath and Morgoth's defeat at the end of the First Age, just two of the original oath-takers survived, Fëanor's eldest sons Maedhros and Maglor. The former put forth a plan that the latter reluctantly accepted. They stole the Silmarils from the camp of the victorious host, but because of the evils they had committed in recovering them, they found that they could no longer touch the holy Jewels without enduring searing pain. In despair, Maedhros threw himself and his Silmaril into the depths of the Earth, while Maglor cast his into the deep ocean. Maglor was the last of the oath-takers, and was said to wander the shores of the world lamenting his pain and loss.