The work is a fictional essay dealing with the preamble to the events described in Tolkien's novel 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. When Tolkien died in 1973, his son, 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; neither divulges 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.
The Elves of Eregion helped Sauron forge many rings, including nineteen Rings of Power. But Sauron had deceived them, for in secret he made the One Ring for himself, which was the master of the rest. These rings of power were distributed to the leaders of the free peoples of Middle Earth in order to achieve control and therefore power. Seven to the Dwarf Lords, nine to mortal Men and three left to the Elves.However Sauron's plan failed: the elves discovered his plot, for as soon as The Dark Lord put his ring on the Elves knew they had been deceived. They removed and hid the three Elven rings and were therefore shielded from it's and his influence, which would have held dominion over all had they kept them. Sauron then waged war upon the Elves in retaliation but, the three Elvish rings remained hidden from him. While many Elves were killed and the kingdom in Eregion destroyed, the Men of Númenor helped the elves and repelled Sauron. After many decades of war, the Men of Númenor decided to capture Sauron and demonstrate their might. As it is described in Akallabêth, Sauron was brought to Númenor as a slave; however, he soon corrupted most Númenóreans, encouraging them to replace their traditional reverence for Eru 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, a cataclysm far beyond the premeditated schemes of Sauron who himself was cast down 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, their ships washed ashore upon the huge waves which battered the coasts and were accepted as rulers in Middle-earth. They unified realms which were already peopled and governed in Númenórean 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's spirit survived the disaster too, and although he had lost his fair physical appearance, he returned safely to his stronghold of old in the land of Mordor and regained his strength.
Years passed, and Sauron, who had renewed his might, gathered his allies of Orcs and evil men, including the Black Númenóreans (most of whom now followed leaders who wore Rings Of Power) and decided to attack the new realms while they were still weak. His onslaught failed, however and King Elendil, his sons, and the Elven kings forces 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. Sauron himself came forth to challenge the Kings of Middle Earth who stood with their Elvish friends. The mighty high king of the Elves, Gil-galad challenged Sauron to a duel, and lost. Then Elendil fought with him and fell as he struck down The Dark Lord; In doing so Isildur's body fell upon his own sword and it shattered beneath his body. Isildur, Elendil's son quickly approached Sauron's body and cut the ring finger off Saurons hand the One Ring and all using the hilt shard of his fathers sword, the final death blow as it would seem. Sauron's spirit departed his dark armored body. Lord Elrond Half Elven who stood nigh tried to convince Isildur to destroy the ring then and there in the fires of Mount Doom where it was made, and the only place it could be destroyed for ever: But he took it for his own and declared that it would be a weregild for his Father, and Brother Anárion, who also was killed during the siege of Barad-dûr). The Elves could now wear their three Rings Of Power freely as The One was no longer worn by the defeated Dark Lord. Thus began the Third Age of Middle-earth.
In the first months of the Third Age Isildur and his guard were ambushed by a band of orcs nigh to The Gladden Fields. With almost all of his men killed Isildur put the Ring on his finger in the hope of alluding the foul Orcs as the Ring made him invisible. He plunged into Anduin the great river in order to escape. Yet the Ring betrayed him. It slipped from his finger revealing him to the Orcs who shot him with many arrows. As Isildur died the Ring was lost in the great river for many centuries.
The Heirs of the remaining royal blood line 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 Númenóreans descent. They became the Rangers of the North, protecting the paths of the North from the menace 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, known as the Istari, had come to Middle-earth: Curunir (Saruman), Mithrandir (Gandalf), Radagast, and others of the Istari who went into the east of Middle-earth. 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 to 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 middle of a giant forest called Mirkwood. During the attack, the force had fled to Mordor. It was Sauron, who was previously thought to have perished. And in the same year, the One Ring was found. Sauron made war on Middle-earth again. In this war, the Heir of Isildur,Aragorn II Elessar, arose and led the host of the West to victory at the Battle of the Pelennor Fields, while Curunir is defeated and captured at Isengard. Then Mithrandir, the sons of Elrond and the Heir of Isildur lead the armies of good to the Black Gate of Mordor and engage the superior Mordor armies. But Frodo the Hobbit went to Mount Doom and destroyed the Ring, defeating Sauron.
After this, it was made clear that Gandalf bore the Red Ring, Narya. Galadriel and Elrond had the other two rings. And the Third Age ends with the Keepers of the Three Rings departing over the Seas, bringing an end to the Eldar except in story and song.