It's a long read, but give it a try. While perhaps far-fetched, I always held an interesting theory on Paarthurnax's motivations for aiding the Dragonborn and the role Alduin's return may have played in his Grand Scheme. I thought it'd be interesting to write this down as a blog post, to trigger a discussion and to hear what others think.
The theory itself
The theory approaches the concept that Paarthurnax's benevolence during the events in 4E year 201 may not at all have been as altuiristic as he has led us to believe. In short, Paarthurnax would gain greatly from Alduin's defeat in more than one way, something he could only achieve through planning and scheming for centuries, perhaps even millennia. After gaining Kyne's trust and during those thousands of years, he would careful seed his influence within ancient groups such as the Greybeards and Thu'um users and through those centuries could have influenced those groups sufficiently to alter their school of thinking in accordance with his greater plans involving Alduin's defeat.
Eventually he will instigate mankind's rebellion, backed by Kyne, against the dragons by teaching mankind how to use the Thu'um. Siding with the Thu'um users, Paarthurnax installs himself as the grandmaster of the Greybeards when Jurgen Windcaller creates the ancient order. It is unknown if Paarthurnax would have whispered the concept of the Greybeards into Windcaller's ear, if Windcaller simply got the idea on his own account and Paarthurnax found the creation of the Greybeard order convenient, or if the subversion of the Thu'um users from militants into pacifists was part of a deal struck with Kyne (who herself is often known to be merciful and gentle).
Let us first look at some of the events Paarthurnax may have manipulated:
The banishment of Alduin: The Elder Scroll on top of the Throat of the World.
Being discontent with Alduin's desire to merely destroy all that is not dragonkind, Paarthurnax desires to remove Alduin from his position and searches for a way to end Alduin. Failing to find someone capable enough of destroying Alduin at the time, he instead seeks for a way to stall Alduin's campaign to rid Mundus of all things that are not dragonkind. Learning about the Elder Scrolls, possibly through Kyne or with knowledge inherited from his father Akatosh, he plans to trap Alduin so that he can buy his time to plot a take-over of all dragons.
When Felldir the Old is taught about the Elder Scroll (possibly, but not per se, by Paarthurnax) he uses this to trap Alduin in time. This created a time wound, a sort of doorway into a tunnel or space in time which trapped Alduin, created on top of the Throat of the World. That very wound would remain where it was formed and would serve it's purpose later when the Dragonborn was to look back in time with another Elder Scroll to learn the Dragonrend Shout. But this time wound would serve Paarthurnax as well. Likely, Paarthurnax stayed in the shadows during this event, keeping Alduin in the dark about his treachery for the moment.
The return of Alduin: The link between the Dragonborn and Alduin.
When the Elder Scroll was first used on top of the Throat of the World, a time wound was created. The wound can be seen as a one-way doorway into this time trap, a door which Alduin was pushed through when the Elder Scroll was used. When the Dragonborn chooses to stop Alduin, an Elder Scroll is used once again at Paarthurnax's advice to allow the Dragonborn to learn the Dragonrend. Just like how the Elder Scroll created a doorway the first time it was used, it would create a doorway this second time as well. Except this time, that doorway is a way-out for Alduin who can return to the world of Men. Something that Paarthurnax could have anticipated and may have planned on with intent, in secret.
Skyrim (the game) explains to us that time is not linear and that dragonkind understand time so well, that they can observe it outside of the chronological order humankind adheres to. This would mean that Alduin did not have to exit the time wound at the moment that the Elder Scroll was used on top of the Throat of the World a second time, but that he could leave whenever he'd wish. Alduin understood well that to ensure his freedom, the Dragonborn MUST survive, because if the Dragonborn were to die prematurely, the Elder Scroll would not be opened a second time at the time wound and thus Alduin would not have been able to escape (modern science calls this the grandfather paradox, where if you kill your (grand)father by traveling back in time before you are born, you erase your own existance). And so Alduin had to think of a point and place in time where he would have enough time to resurrect his dragon army, while at the same time ensuring the Dragonborn's survival. The most beneficial moment for that would be during the Dragonborn's imminent execution at Helgen: at that moment the Dragonborn was still weak and still ignorant of the power of the Thu'um, thus Alduin could bring back the dragons relatively undisturbed for quite a while. He would plan to destroy the Dragonborn as soon as the Elder Scroll was used, a plan which indeed he acted upon. Only at this time would Paarthurnax join the battle.
Alduin would thus run into the nasty surprise of having to face off against both Paarthurnax AND a dragonborn who has been trained to use the Thu'um more efficiently than Alduin could ever dream of. Worst of all, this Dragonborn already knew how to use the Dragonrend and was by far not as much of a pushover as the Thu'um users Alduin faced and defeated in the past. With Alduin choosing to retreat and lick his wounds in Sovngarde, Paarthurnax knew that the Dragonborn could stop Alduin near permanently by following him into Sovngarde and fighting Alduin while he was still weakened, which indeed did result in Alduin's defeat.
The theory concluded
So with Alduin defeated, Paarthurnax chooses a path which is less destructive than that of Alduin. He wishes to teach the way of the voice to other dragons so that they can live in a more peaceful coexistence with mankind. We have seen after all that the dragons became near-extinct the last time they faced the wielders of the Thu'um and a less aggressive approach may ensure that the dragons survive long enough for Paarthurnax to assume a sufficient control over them that he can be considered to wield considerable power. All he would have to do for this is biding his time:
First of all the Dragonborn is still mortal and thus will die of old age. When that day comes, there is no-one strong enough to dispose of the dragons with as little effort as the Dragonborn could, and so the Dragonborn is best kept as a friend, rather than as an enemy.
Second of all, with Alduin gone and the Dragonborn having fulfilled their part in the prophecy, the Greybeards may cease to exist. This would ensure that the Thu'um would once again be forgotten and that no-one could face a powerful dragon threat.
What does this mean? If this theory proves to be true, Paarthurnax truly doublecrossed everyone: Alduin, the Greybeards, the Dragonborn, the dragons. But even if he did manipulate everyone and everything, he may still hold a benevolent intent. Humankind gets to survive, and so does dragonkind, both benefit. He may indeed prefer a world of shared philosophy and scholarship, but if the theory is true he is true to his roots of being a dragon, lying and manipulating to achieve this and trusting no-one but himself with the execution of this plan, although he has come far enough to not think all that black-and-white as Alduin did.
Basically, he took a gamble and even hinted at the Blades having good reason to doubt a dragon. By playing 'open and truthful' he hoped to gain the Dragonborn's sympathy so that he would be spared, but he himself contradicts himself in that he may still lie. He could have died if the Dragonborn did not believe him and he may have if you, the player, chose to do just that. So don't open up those old saves to kill Paarthurnax: he may be an unpredictable playing card, but he's also the ace in the hole. There are a lot of worse beings that could be in charge of the remaining dragons out there.
Why did he act now? Tiber Septim could have done the same as the Dragonborn.
Indeed he could have chosen to execute his plan centuries earlier by bringing back Alduin the same way he did now and having Tiber Septim facing off against Alduin instead, but Septim was required to create the Empire and thus he was left alone to do so. It is possible that Paarthurnax may have even been the one who spoke the prophecy of Tiber Septim's greatness and ascension to the Emperor that united an Empire. But wether or not Paarthurnax and Tiber Septim ever met remains unknown, but since Paarthurnax as grandmaster of the Greybeards also taught the Dragonborn from Skyrim, it is definately possible. Without Tiber Septim's rule mankind may have ceased to exist a long time ago.
And what if Alduin IS Akatosh?
And if indeed Alduin would have been a manifestation or rebirth of Akatosh in some way, like many scholars like to believe, then Paarthurnax just took over the position held by the head chief deity of all the divines. And that's a lot of power. It would explain how Alduin understood time well enough to plan his rescue of the Dragonborn, and the latters failed death the moment the time wound was touched a second time with an Elder Scroll. But as all dragons are capable of understanding time in much greater propertions than humankind, it's not likely that Alduin and Akatosh are the same. If they were, Alduin's demise is truly catastrophical.
Your thoughts? I think it's a scary theory, or atleast for me it caused a major 'that clever, clever Paarthurnax!' reaction.