When the elves had moved to the land of Aman (Garden of Eden), the Valar (Tolkien's pantheon) and the Maiar taught them many things. Unto Curufinwe was given the most skill and he created the Silmaril's after much secret labour. In them was trapped the light of the trees of Valinor (Telperion and Laurelin) and Morgoth lusted after them and finally smote the trees and stole the casket from Finwe after killing him.
Then Feanor and his 7 sons swore that oath - "Then Feanor swore a terrible oath. His seven sons leapt straightway to his side and took the selfsame vow together, and red as blood shone their drawn swords in the glare of the torches. They swore an oath which none shall break, and none should take, by the name even of Iluvatar, calling the Everlasting Dark upon them if they kept it not; and Manwe they named in witness, and Varda, and the hallowed mountain of Taniquetil, vowing to pursue with vengeance and hatred to the ends of the World Vala, Demon, Elf or Man as yet unborn, or any creature, great or small, good or evil, that time should bring forth unto the end of days, whoso should hold or take or keep a Silmaril from their possession."
And he swayed his people into departing from Aman, to pursue Morgoth back to Middle Earth and reclaim their Silmarils. But as they went, a messenger from Manwe bid them stop and come back "for the hour is evil, and your road leads to sorrow that ye do not foresee", but Feanor laughed and said "'Say this to Manwe Sulimo, High King of Arda: if Feanor cannot overthrow Morgoth, at least he delays not to assail him, and sits not idle in grief. And it may be that Eru has set in me a fire greater than thou knowest. Such hurt at the least will I do to the Foe of the Valar that even the mighty in the Ring of Doom shall wonder to hear it. Yea, in the end they shall follow me. Farewell!'"
"The works of wonder for the glory of Arda that he might otherwise have wrought only Manwe might in some measure conceive. And it was told by the Vanyar who held vigil with the Valar that when the messengers declared to Manwe the answers of Feanor to his heralds, Manwe wept and bowed his head. But at that last word of Feanor: that at the least the Noldor should do deeds to live in song for ever, he raised his head, as one that hears a voice far off, and he said: 'So shall it be! Dear-bought those songs shall be accounted, and yet shall be well-bought. For the price could be no other. Thus even as Eru spoke to us shall beauty not before conceived be brought into Ea, and evil yet be good to have been.'"