"In those days the Great Enemy, of whom Sauron of Mordor was but a servant, dwelt in Angband in the North, and the Elves of the West coming back to Middle-earth made war upon him to regain the Silmarils which he had stolen"
GNU Free Documentation LicenseMorgoth - Credit: Outcast/Wikimedia Commons
The Great Enemy is Morgoth, an evil Vala who stole the Silmarils from Valinor and poisoned the Two Trees that gave light to the island (see bookmark for page 104).

The Two Trees
Creative Commons Attribution Share AlikeThe Two Trees - Credit: Julia Pelzer at Ardapedia
The Silmarils were captivating jewels made by Fёanor (one of the Noldorin Elves who came to Valinor) that were able to hold inside them some of the light from the Two Trees. When Morgoth stole the jewels and poisoned the Trees, Valinor was left in perpetual twilight. The Noldorin Elves were furious, and swore an oath to return the Silmarils. The Valar did not wish the elves to leave, but Fёanor and the Noldor disobeyed them, causing the exile of their people from Valinor. After this the Undying Lands were closed to the elves.

Fёanor and the Noldor returned to Middle-earth and fought a war against Morgoth that lasted for centuries, during which Fёanor was slain. One Silmaril was recovered by Beren and Lúthien, and brought back by Eärendil the Mariner. Eventually, the Valar took pity on the people of Middle-earth and came themselves to destroy Morgoth. In the ensuing battle, Morgoth’s fortress of Thangorodrim at Angband was destroyed and the Great Enemy defeated. The Silmarils were recovered, but two were lost forever, one beneath the earth and one in the sea. The one that had been recovered by Beren and Lúthien was placed in the sky by Elbereth as a symbol of hope for the people of Middle-earth. At the beginning of the Second Age the Noldorin Elves were allowed to return to Valinor, but many chose to remain for a time in Middle-earth, becoming known as High Elves (see bookmark for page 104).

Note: Fёanor also created the palantírs, or seeing-stones, that will appear later in Lord of the Rings, and perfected the Tengwar script used by the elves (see bookmark for page 66).