The Oath of Fëanor was an oath taken by the Elf Fëanor and his seven sons to non-negotiable enmity against any person who would take a Silmaril or keep it from them, early in the First Age. Once Morgoth killed Finwe and stole the Silmarilli, this oath became the cause of some of the most terrible and tragic crimes in the history of Arda.
They swore an oath which none shall break, and none should take, by the name even of Ilúvatar, calling the Everlasting Dark upon them if they kept it not...
—The Silmarillion: Quenta Silmarillion, "Of the Flight of the Noldor"
Learning of his father's death at the hands of Morgoth, Fëanor went back to Tirion, breaking the exile set by the Valar. At Tirion, Fëanor gave an impassioned speech in which he convinced the vast majority of the Ñoldor to follow him to Middle-earth to wrest the Silmarils back from Morgoth and avenge Finwë. At the end of his speech, furious and ever prideful, Fëanor swore the dreadful oath to Eru, calling Manwë and Varda in witness. This oath saw him pledge, on pain of calling upon himself the Everlasting Darkness, to pursue with violence any who would keep or hold a Silmaril from him, no matter who it was, and no matter what the reason or circumstance. His seven sons leaped to his side and swore the oath with him. 
Fëanor died soon after his return to Middle-Earth but the remaining oath-takers, his sons, lived on in relative harmony with the Eldar of Beleriand, in no small part thanks to the diplomatic skill of Maedhros.  For the greater part of the First Age, the oath kept the sons of Fëanor united in one cause: to defeat Morgoth and retrieve the Silmarils, for so long as the jewels remained in the hands of the Dark Lord, they were irretrievable by force. And so the oath slumbered, as by fighting Morgoth, even hopelessly, the brothers were still technically attempting to fulfill it.
However, the terrible dangers of the oath became apparent when Beren and Lúthien managed to recover one of the jewels from Morgoth. Now that the jewel was no longer in Morgoth's hands, it became possible for the brothers to retrieve it and genuinely fulfill the oath. Beren gave the Silmaril to King Thingol of Doriath who had demanded it as a bride-price for his daughter, believing that Beren could not possibly retrieve one. When the sons of Fëanor learned of this, they demanded its surrender. However, Thingol was insulted by the demand and had begun to lust for the Silmaril. He refused their request, and Curufin and Celegorm openly threatened to destroy Doriath. This threat was the cause of considerable turmoil and resulted in much grief, for Maedhros was attempting at the time to forge a coalition between the various Elven kingdoms to engage in a final, decisive strike against Morgoth's power. The threat caused Thingol to close his borders and send no aid to the military alliance. Though the Elves would not have had the power in any circumstance to overthrow Morgoth, the loss of Thingol's aid proved detrimental to the alliance. After the Elven defeat during Nírnaeth Arnoediad, a series of unfortunate events led to the death of King Thingol and the disappearance of Queen Melian, who returned to Valinor in grief. With her departure, the Girdle of Melian, which had protected Doriath from attack, faded. Beren and Lúthien's son, Dior, was crowned King of Doriath. He likewise refused to surrender the jewel, and the brothers attacked Doriath and sacked the halls of the Sindar, committing the second kin-slaying. However, the Silmaril escaped the destruction of Doriath and the oath drove the sons onwards. Eventually, they learned that it it had come into the possession of Elwing, who dwelt near the mouths of Sirion. They attacked the and slaughtered the defenseless exiles of Gondolin and Doriath, committing the third and most terrible kin-slaying.
But the Silmaril escaped them again and was borne by Eärendil into the West. That Silmaril was forever lost to the sons of Fëanor but two more remained in the crown of Morgoth. With the complete destruction of the Elven kingdoms of Beleriand, it seemed impossible that these would ever see the light of day again. However, Eärendil convinced the Valar to take pity on the Elves and Edain under Morgoth's power, and initiated the War of Wrath. This war saw Morgoth's power utterly overthrown, and the two remaining Silmarils were wrested from him. However, most of the sons of Fëanor had been slain in battle, and no few of them during the kinslayings by their victims. The only brothers to survive the third and final kinslaying were Maedhros and Maglor. They demanded the Silmarils from Eönwë, the standard bearer of Manwë, but because of the wicked deeds they had committed in the name of retrieving it, they were denied. By now sick at heart of keeping the dreadful oath, neither particularly wanted anything more to do with the Silmarils, and Maglor openly opposed attempting to retrieve them. However, Maedhros convinced him to help him lay hands upon the jewels, claiming that because of the oath, they simply had no choice. They snuck in to the camp of the victorious Host of the West, slew the guard around the jewels, and retrieved them. The camp was roused against them, and they prepared themselves to die defending their claim. But Eönwë simply permitted them to leave unharmed. However, due to the terrible deeds committed by the brothers in their retrieval of the Silmarils they found they could not handle the Silmarils without enduring searing pain. The two brothers parted and in his anguish Maedhros threw himself and his Silmaril into a fiery chasm. Maglor on the other hand tossed his Silmaril into the sea and was said to wander the coast forever after lamenting his loss and pain.
The Oath itself, as given in Morgoth's Ring, is as follows:
“Be he foe or friend, be he foul or clean
Brood of Morgoth or bright Vala,
Elda or Maia or Aftercomer,
Man yet unborn upon Middle-earth,
Neither law, nor love, nor league of swords,
Dread nor danger, not Doom itself
Shall defend him from Fëanáro, and Fëanáro’s kin,
Whoso hideth or hoardeth, or in hand taketh,
Finding keepeth or afar casteth
A Silmaril. This swear we all…
Death we will deal him ere Day’s ending,
Woe unto world’s end! Our word hear thou,
Eru Allfather! To the everlasting
Darkness doom us if our deed faileth…
On the holy mountain hear in witness
and our vow remember,
Manwë and Varda!" 
|Foreign Language||Translated name|
|Afrikaans||Eed van Fëanor|
|Albanian||Betimi i Fëanor|
|Belarusian Cyrillic||Клятва ў Феанор|
|Bulgarian Cyrillic||Клетва на Феанор|
|Catalan||Jurament de Fèanor|
|Cebuano||Panumpa sa Feanor|
|Dutch||Eed van Fëanor|
|Esperanto||Ĵuro de Fëanor|
|Filipino||Panunumpa ng feanor|
|French||Serment de Fëanor|
|Galician||Xuramento de Fëanor|
|Hindi||फैनोर की शपथ|
|Italian||Giuramento di Fëanor|
|Kazakh||Феорордың анты (Cyrillic) Feorordıñ antı (Latin)|
|Kyrgyz Cyrillic||Фэанордын ант|
|Macedonian Cyrillic||Заклетва на Феанор|
|Maltese||Ġurament ta Fëanor|
|Mongolian Cyrillic||Фэанор-ийн тангараг|
|Nepalese||ॐअथ ओफ़ फ़नोरे|
|Romanian||Jurământul lui Fëanor|
|Serbian||Заклетва Фëанор (Cyrillic) Zakletva Fëanor (Latin)|
|Spanish||Juramento de Fëanor|
|Swahili||Kiapo cha feanor|
|Tajik Cyrillic||Оатҳ оф Феанор|
|Persian||واته وف فهانور|
|Polish||Przysięga synów Fëanora|
|Portuguese||Juramento de Fëanor|
|Ukrainian Cyrillic||Присяга Феанора|
|Urdu||فیناور کا ہالہف|
|Uzbek||Феанорънин Қасамёди (Cyrillic) Feanor'nin Qasamyodi (Latin)|
|Welsh||Llw o Fëanor|
|Yiddish||ױאַטה אָף פֿעאַנאָר|
- The Silmarillion, Quenta Silmarillion, Chapter IX: "Of the Flight of the Noldor"
- The Silmarillion, Quenta Silmarillion, Chapter XIX: "Of Beren and Lúthien"
- The Silmarillion, Quenta Silmarillion, Chapter XXII: "Of the Ruin of Doriath"
- The Silmarillion, Quenta Silmarillion, Chapter XXIV: "Of the Voyage of Eärendil and the War of Wrath"
- The History of Middle-earth, Vol. 10: Morgoth's Ring, The Annals of Aman