In the Elder Days Morgoth, the first Dark Lord, dwelt in Middle-earth and the High Elves made war on him for the recovery of the Silmarilli - those perfect jewels created by Feanor, most gifted of the Elves.
The Silmarillion tells of the rebellion of Feanor and his kindred against the gods, and of the war, hopeless despite great heroism, against the great Enemy.
This is the ancient history of Middle-earth, the foundations of the world and its peoples before the great events recorded in The Hobbit and The Lord of the Rings.