Spoiler warning! Everyone who plays Oblivion: Remastered is immediately confronted with the death of the Emperor. But who was he anyway and what is the Mythic Dawn up to?

Spoiler warning! Everyone who plays Oblivion: Remastered is immediately confronted with the death of the Emperor. But who was he anyway and what is the Mythic Dawn up to?
Why did the Mythic Dawn assassinate the Emperor? And who is Mehrunes Dagon anyway? Whether you're a veteran who has wandered Nirn hundreds of times or a complete newcomer to The Elder Scrolls, here's roughly everything you need to know to fully enjoy the story of Oblivion.
Disclaimer
The story of The Elder Scrolls is mainly told through books or scrolls you find in the game. This also means that there are theories that do not match 100% with other sources in the game. In this article, however, we will focus mostly on confirmed information that holds up throughout the story. We also leave out a lot of details and generalize a lot, otherwise this article would turn into a novel.
Who Are Aedra And Daedra?
The story behind it is explained in the game by the followers of various religions, just like in real life. Basically, however, most creatures more or less follow this story.

The story of Elder Scrolls takes place on the planet Nirn. This exists on the "mortal realm" called Mundus. Most agree that the planet was created out of the Void by the creator of all gods, Lorkhan. Many gods helped him to create this world, known as Aedra in the Elder Scrolls universe. You can also find them in the game in various churches, such as Mara, Dibella, Julianos and so on.
However, when the gods realized that forming Nirn drained some of their powers, some of them split off. These are called Magna-Ge. On leaving, these gods tore holes in the dimension of Mundus and created the stars. However, there were also gods who did not want to help create the planet in the first place. These are called Daedra.

The Ways Of The Mythic Dawn
As in reality, there are cults who beg to differ.
The Mythic Dawn is not alone in this view, as there are other believers in Daedric princes. Instead, this cult believes that Lorkhan was the original Daedric Prince and that Aedra are the 'anti-gods' in that they usurped Lorkhan's creation and the Magna-Ge fled as a result.
As a result, Lorkhan's creatures had to part with his immortality and became mortal. These creatures are actually us, the humans and creatures who now live on the world of Nirn.
According to the Mythic Dawn, the Aedra have betrayed Lorkhan and thus ensured that we cannot be immortal now. The Daedric prince Mehrunes Dagon is now supposed to help them with their revenge.

Who Is Mehrunes Dagon, And Why Does He Open All These Gates?
The Mythic Dawn believes that Mehrunes Dagon, the Prince of Chaos and Destruction, was created by the Magna-Ge as a kind of counterpart to Lorkhan. They also believe that the only way out is for this prince to walk the earth of Nirn and "repair" everything so that Nirn becomes what Lorkhan once imagined it to be. But where does this theory come from?

Mankar Camoran And His Commentaries
Mankar Camoran's father once tried to conquer the world of Nirn with a Daedric army, and now he is the one trying to do so. By becoming so loyal to Prince Mehrunes Dagon, the latter has written him the Daedric artifact "Mysterium Xarxes".
This artifact is so powerful that "normal" creatures can barely read it. Mankar Camoran wrote his four books "Commentaries on the Mystery of Xarxes", which became the basis for the Mystic Dawn faith.
He has also learned to create his own small realm within Mehrunes Dagon's own. This is supposed to be a paradise for the followers of the cult where they learn to rule the new world.

Why Do They Have A Problem With The Septims?
To cut a long story short: Ever since the slave queen Alessia captured her soul in a piece of jewelry, the "Amulet of Kings", one of her descendants can use it to ignite the dragon fires, which in turn hold a barrier between Mundus and the plains of Oblivion. So it is clear that the ruler Uriel Septim and all his descendants must disappear from the scene until Mehrunes Dagon can enter Nirn.
The Elder Scrolls IV: Oblivion
And we have already arrived at the beginnings of the game. After the Mythic Dawn decided to put their plan into action, the members of the cult murdered Uriel Septim's descendants in various ways. If you've played Morrowind before, you'll know that Uriel's favorite pastime is dreaming about the player character and sending them on insane journeys. So that's the case this time too.

Spoilers for the first part of the story await you here!
We meet the Emperor in our prison cell. Here we learn that he dreamed of our beautiful face and that his time has come. He instructs us to take the amulet to Jauffre, the leader of the "Blades" — the ruler's personal Secret Service and bodyguards. And he tells you a well-kept secret: he has another illegitimate son.
Jauffre tells you that you will find Martin Septim in Kvatch, where he lives in seclusion as a priest, completely unaware of his special blood. Once you arrive, however, you realize that it might be too late. Instead of a quiet town, you find bloody chaos and a huge burning gate that Uriel Septim has already seen in a dream: it is a passageway to the cruel plane of Oblivion.
Martin Septim
After we have closed this gate to hell, we find him in the church in the now destroyed Kvatch. But what do we know about Martin?

Martin Septim is the child of Emperor Uriel Septim and a palace servant, about whom we know little.
Uriel asked his friend and leader of the Blades, Jauffre, to keep the child safe and leave him in the dark about his true origins. Martin discovered a talent for magic at an early age, which is why he was invited to join the Mages' Guild in Kvatch.
With friends he made there, he was frustrated with the restrictions of the Mages Guild and decided to worship the Daedric Prince Sanguine (who is the Prince of... drinking and partying? And whose rituals involve... adult acts).

After an accident led to the death of his friends, Martin renounced the Daedra and became a priest in the city of Kvatch. But this is where his life and the lives of all the inhabitants of Nirn changed: the Mythic Dawn knew nothing of Martin, and so he became the only one who could reignite the dragon fires and save Nirn.
And now, we release you into the world of The Elder Scrolls Oblivion: Remastered. Do you think I've left out a critical detail, or do you have a different view of the events? Then feel free to write it in the comments. But please don't open a gate to Oblivion when you arrive.