The Elder Scrolls IV: Oblivion describes the events that ended the Third Age of Tamriel. These events take place beginning a few years after the fulfillment of the Nerevarine Prophecies and the defeat of Dagoth Ur. The relative peace of Cyrodiil is threatened by a plot to bring the Daedric Prince Mehrunes Dagon from his native plane of Oblivion to conquer Tamriel.
Just prior to the start of the events in the game, you are imprisoned in an Imperial cell for unspecified (and presumably larger) crimes. A daedric cult known as the Mythic Dawn has tracked down and assassinated each of the heirs to the Imperial throne, and are finally moving in on the Emperor Uriel Septim VII. The Emperor's escape route passes directly through your cell, giving you the opportunity to escape the prison in his wake.
In the process of your escape, the Emperor recognizes you from a dream and predicts your importance in the coming days. Soon after, the Mythic Dawn succeeds in assassinating Uriel. With no heir to take his place, you soon learn the purpose behind the Mythic Dawn's plan: with the death of the last known Septim and no one to take the throne, the mystical barriers in place to keep the Daedra confined to Oblivion are gone. Your part in these events begins when, just prior to his death, the Emperor charges you with finding the last hidden heir, protecting the Amulet of Kings, and stop the Mythic Dawn and Mehrunes Dagon before they can conquer all of Tamriel.
According to popular legend, before the advent of the mortal plane, Nirn, the universe was populated by a variety of ethereal, immortal beings. Some of these beings had a strong enough presence to develop their own identities, with distinct names and personalities. These beings are what the mortal inhabitants of Tamriel worship as gods.
At some point, one of these beings, Lorkhan, proposed the idea of creating the physical plane, and populating it with lesser, mortal races. Some of his peers went along with his plan, and together gave birth to Nirn and its inhabitants. Others chose to distance themselves from Lorkhan's plan, believing it to be a trick. Thus, the inhabitants of this new plane, including the races of mer and men, divide the ethereal beings into the Aedra ("our ancestors") and the Daedra ("not our ancestors"). The mortal races would eventually elevate these beings to the position of gods, primarily worshipping the Aedra, but occasionally (most notably, the Chimer and Dunmer) worshiping powerful Daedra Princes.
Before the rise of the human empires, during the beginning of the first era, the area now called Cyrodiil was ruled by a race of Mer known as the Ayleids. Like the other races of Mer, the Ayleid pantheon consisted primarily of powerful Aedra. However, the Ayleid race were also known to employ the daedra, both the lesser daedric creatures and the more powerful Daedra Princes, to maintain their dominance over the other races.
In the year 1E 143, the human King Harald drove the mer out of Skyrim and established the first human kingdom on Tamriel. Approximately a hundred years later, in 1E 242, the human slaves in Cyrodiil revolted, led by the Slave Queen Alessia. With the help of the Nords in Skyrim, Alessia succeeded in driving the Ayleids out of Cyrodiil into refuge in Valenwood, and establishing the human kingdom of Cyrodiil. Alessia constructed a new pantheon of gods that combined Aedra from both Ayleid and Nordic traditions, creating the Eight Divines. The divine being Shezarr (another name for Lorkhan, considered the patron god of man and an enemy of the mer) soon arrived in White Gold Tower and presented Alessia a gift to help preserve peace in Cyrodiil: the Amulet of Kings. He bound Alessia's soul into this amulet, elevating her to the position of the first saint of Cyrodiil, and used the amulet to seal off the planes of Oblivion, where the powerful Daedric princes made their home, from Tamriel.
Several thousand years later, near the end of the First Era, Cyrodiil is invaded by beings from the remote continent of Akavir (e.g. Pale Pass). This invasion unites the eastern and western segments of Cyrodiil to drive the invaders back, and begins the Second Empire of Cyrodiil, the Reman dynasty under King Reman I. It is King Reman who begins the tradition of lighting the dragonfires on the crowning of a new King of Cyrodiil, and extinguishing them magically upon his death. This tradition is bound to the passing of the Amulet of Kings to each successive member of the royal bloodline, and thus the protection of Cyrodiil from daedric invaders.
The Third EraEdit
Approximately 800 years into the Second Era, the king of Cyrodiil, Tiber Septim, begins a military conquest of the other independent kingdoms of Tamriel into a single unified empire. Around 2E 896, Septim negotiates a treaty with the Tribunal of Morrowind, the last truly free nation, by which Morrowind would become an Imperial province but maintain a high degree of independence. In return, Vivec provides magical assistance to Septim, allowing him to complete his conquest of Tamriel, and begin the Third Era. Upon his death, Tiber Septim is elevated to the status of a divine being, becoming the ninth divine, Talos.
Septim's bloodline continues for several hundred years, though not without it's share of strife. A number of civil wars among the Septim line occur, but each time, a true blooded descendant of Tiber Septim takes the throne and the dragonfires remain lit. In 3E 388, Emperor Uriel VII Septim is crowned, upon the death of his father, Pelagius SeptimV Septim. The power of the empire had begun to wane during the previous kingdoms, but Pelagius, then his son Uriel, were dedicated to reunifying the empire. During his reign, Uriel VII survived an attempt by Jagar Tharn to imprison him and assume control, an attempt by Mehrunes Dagon to destroy the Imperial Battlemages, an attempted revolt involving the murder of the King of Daggerfall, and the near-destruction of Morrowind by an ancient enemy of the ruling Tribunal.
In 3E 433, Uriel VII's sons are murdered by a cult known as the Mythic Dawn, and then Uriel himself.