The Return of Alduin is the Work of Akatosh

Akatosh and Alduin. Two Gods of immense power, two Gods that have changed the course of history forever, two Gods with a burning hatred for one another, a hatred matched not once by any other pair in history. Akatosh hated Alduin, and perhaps it was so, that at the height of Alduin's power, which is by no means randomly when he was at the height of his worship, Akatosh couldn't stop him. Akatosh could not take Alduin and claim his soul for his own, because Alduin's power, and quite possibly his shard, was superior, effortlessly so. The Dragon cult was strong, the spirits of men disheartened in an event many would presume to be the end of everything, and when Kynareth was their primary hero. Akatosh was weak. Alduin was strong.

And now we get onto the issue of the Elder Scrolls. First off, this is based on the presumption that the Aedra did indeed write the elder scrolls

Time is not linear for Akatosh, so he could easily have written the Elder Scroll regarding this prophecy very recently.

Or, he could've foretold these events and formulated his plan when he made the elder scrolls, alongside the other Aedra.

So, he'd need to pick the best time, where he is most powerful. My basic theory is Akatosh is as strong as the time he's watching over and influencing. So, Akatosh needed to pick a good time. And so, he chose an extremely convenient era, for several reasons:

  1. Talos worship has been torn to shreds, so each of the 8 divines will have a larger share of focus.

  2. He's very recently saved the world with Martin Septim, from the Oblivion crisis.

  3. Akatosh/Auriel worship is strong, all over Tamriel. Not the strongest time, but certainly adequate given the other beneficial scenarios.

So, even if you ignore my no linear time theory, Akatosh is more powerful than ever, and guess who's weaker than ever? That's right, Alduin. Now it's time to turn the tables. To pull Alduin out from the prison of time and bring him back. Akatosh is ready.

However, despite this new power. Akatosh can't directly intervene. He can't directly drop into Tamriel itself and destroy Alduin. He needs an avatar, or at least, a soul blessed mortal to stop the World Eater. Enter, the Last Dragonborn. Now, the theory splits off in two ways. The first theory is appropriate if the Dragonborn is an Avatar of Akatosh. So let's start with that.

Akatosh's avatar would no doubt not be powerful enough to absorb Alduin's soul. Akatosh would have to do this personally. However, he can't enter the mortal plane with enough power to be able to take him. So, it has to be done in Akatosh's own plane, Aetherius. And it just so happens that Alduin is defeated in Aetherius, in Sovngarde, and the LDB does not absorb the soul. This makes sense, no?

And the second branch, is, if the LDB is just a champion or merely blessed mortal. In which case, then by no means would he be able to truly absorb the soul. Now, this LDB, if not following the sub conscious instinct of a God, is following the words of Paarthurnax. A fellow friend of man, and a Dragon. Maybe Akatosh has only allowed Paarthurnax to live this long to guide the LDB, but would be happy to see him gone, or perhaps even compelled, to have this former of ally of Alduin gone. Does this not explain the irrational and stubborn behaviour of the Blades?

So, Akatosh has every reason to want Alduin's soul. Akatosh is also more powerful than ever, while Alduin is weaker. Alduin is defeated in Aetherius, the location of Akatosh, where it appears that something or someone else has taken the soul. Akatosh is in charge of time, and will thus choose when it ends. Now he truly has that control, and can release Alduin when he sees fit. This fits in with that Arngeir says at the end of the main quest.

So there we go. It all fits together. Here's a link to this: https://www.reddit.com/r/teslore/comments/3n3z66/the_last_dragonborn_is_an_avatar_of_akatosh/?, and this https://www.reddit.com/r/teslore/comments/3nfv5g/all_elder_scrolls_protagonists_are_part_of_a/ to get a bit more insight on my head lore. Thanks for reading and post your thoughts below :)