Of the Rings of Power and the Third Age
The work is a historical essay dealing with the preamble to the events described in The Lord of the Rings, and the events themselves, in the style of The Silmarillion. The fact that those events are explored in a mere handful of pages suggests that if the events described in the rest of The Silmarillion had been written in the style of The Lord of the Rings they would have filled hundreds of volumes.
After Tolkien's death in 1973, Christopher Tolkien completed this part, assisted by Guy Gavriel Kay. '"Of the Rings of Power and the Third Age" bears some similarities to Elrond's narrative in The Fellowship of the Ring during the chapter "The Council of Elrond"; both do not divulge any details about how Arnor was destroyed and how Gondor became kingless. The closeness is perhaps intentional; as Elrond told the Second and Third Age through his eyes, the Silmarillion is supposed to be told through the point of the view of the Eldar.
As the name implies, the events of the essay are focused around magical artifacts: the Rings of Power.
However Sauron's plan failed: the Elves discovered his plot and discarded their Rings until they could be shielded from his influence. Sauron then waged war upon the Elves. During the war, many Elves were killed and the kingdom in Eregion destroyed. Sauron captured all the Rings of Power except the Three and he gave seven of them to Dwarves and nine to Men. But Sauron feared to assail Lindon as the Men of Númenor aided Gil-galad, the mighty elvenking. Throughout the Black Years or Days of Flight Sauron gathered to him all the evil things of Days of Morgoth. Hundreds of years later, the Men of Númenor decided to capture Sauron to demonstrate their might. As it is described in Akallabêth, Sauron was brought to Númenor as a slave; however, he soon corrupted most Númenórians, encouraging them to replace their traditional reverence for Ilúvatar with worship of Melkor, or Morgoth, Sauron's previous master. Under Sauron's influence, the Númenórians decided to challenge the Valar by invading Aman. As a result, Númenor was destroyed and sank beneath the waves.
Only a few survivors left Númenor before it was too late, and led by Elendil the Tall and his two sons Isildur and Anárion, they had settled in Middle-earth. They created realms that were governed in Númenórian style: Elendil ruled over Arnor in the North, and Isildur and Anarion ruled together in the great country of Gondor in the South. However, Sauron survived the disaster, and although he had lost his fair appearance, both he and his One Ring returned safely to his stronghold of old in the land of Mordor.
Years passed, and Sauron, who had renewed his might, decided to attack the new realms while they were still weak. His onslaught failed, however, Elendil, his sons, and the Elven kings fought back. For many years the great coalition (The Last Alliance of Elves and Men, as it became known) besieged Mordor. At last the host broke through to Sauron's fortress Barad-dûr. Gil-galad and Elendil wrestled with Sauron and were slain; however, they managed to defeat Sauron. Isildur, Elendil's son approached Sauron's body and cut off his finger with the One Ring. In vain Elrond and Círdan tried to convince Isildur to destroy the ring in the fire of Mount Doom where it was made. But Isildur took it for his own and declared that it was his and his folk's, a consolation after the enormous loss of the war (besides the death of Elendil, his father, another of those who perished was his brother Anárion, who was killed during the siege of Barad-dûr). Thus began the Third Age of Middle-earth.
Isildur himself died soon in a sudden ambush by a band of Orcs near Gladden Fields, and the Ring that had betrayed him was lost in the great river Anduin. Heirs of royal blood were chosen to lead Arnor and Gondor. For a millennium, both realms enjoyed relative freedom and prosperity. However afterwards, Arnor became subject to attacks from the north-eastern kingdom of Angmar. More and more people fled from the North, and although Angmar was defeated by the beginning of the third millennium of Third Age, Arnor was no more. Its people were scattered, and its royalty decreased in number and fame; however they remained true to their Númenorian descent. They became the Rangers of the North, protecting the paths of the North from the menace that came from the East.
As for Gondor, it prospered for much of the Third Age. However in the beginning of its third millennium, this began to change. Gondor was assailed by Orcs and Men from the nearby Mordor. For a long time, no one suspected that the same force that had driven the attacks upon Arnor was now fighting Gondor.
A thousand years earlier, several Wizards had come to the land: Saruman, Radagast, Gandalf, and two Blue Wizards. Although it was unknown to the peoples of Middle-earth, they were emissaries from the West, sent on behalf of the Valar to help them obtain their freedom. For many centuries they were silent, and little was done by them apart from observation and counsel. However as the times darkened, they decided to take action against a mysterious dark force which seemed to dwell in the fortress of Dol Guldur amidst the forest of Mirkwood. During the attack, the force fled to Mordor and was revealed as Sauron, who was thought to have perished. And in the same year, the One Ring was found.