Not a feat this time, but a counter argument. When debating the power of Dovahkiin, Alduin and Miraak, this quote is often brought up;
So great and terrible were the forces unleashed in this contest that Solstheim was torn apart from the mainland of Skyrim. Here, the myth clearly descends into the realm of pure fantasy.
Miraak and Vahlok tearing Solstheim from Skyrim is stated by the same book to be 'pure fantasy' so this feat isn't reliable. In terms of feats, Miraak and Vahlok scale to much better (and in the case of Miraak, have better feats outright), so make of you will if splitting Solstheim is legit or not. A friend of mine has pointed out there's places where the landscape of Soltheim matches that of Morrowind and Skyrim, so that's some support of the argument in case of (which I may look into later).
However, that is not the issue here. The issue is that the author of The Guardian and the Traitor is being treated as authorative on the subject.
To start with, the name of the author spells out some information for us already. 'Lucius Gallus.' An Imperial, not a Skaal. His account goes on to say this outright; this is the reason he went to the Skaal Village in the first place, to learn more of the myth from the Skaal in person.
Certain that this myth is rooted in history I set out to learn what I could and perhaps piece together a presumptive account of the events that gave rise to the legend.
The tale is remembered best by the shamans of the Skaal, that unique tribe of Nords whose culture evolved along an entirely divergent path than that of their brethren in Skyrim.
I spoke at length to the shaman of Skaal Village, a wise and hospitable man named Breigr Winter-Moon. He described an age long ago when Dragon ruled over the whole world and were worshipped as gods by men. Presiding over this cult of dragon-worshippers were the Dragon Priests, powerful mages who could speak the dragon language and call upon the power of the thu'um, or Voice.
So not only is he hearing this secondhand, but he admits that he came here to learn more of the legend in the first place.
If this is not enough evidence that the source isn't authorative, take further note of what he says in regards to the fight between Miraak and Vahlok...
The Guardian, whom the legend presents as a paragon of loyalty and nobility, finally defeats the despicable Traitor, who seems to represent all that is corrupt and evil in men. Their epic duel is clearly representative of a greater struggle between good and evil. Perhaps it is this timeless quality that has kept the tale alive for so long.
The author says that the fight is meant to be representative of a greater struggle between good and evil. While this still may be the case (TES is a world of Metaphors made Manifest), we can confirm that not only are both Miraak and Vahlok real, but they did infact fight.
There is a better example however; near the end of the book, it says as such...
There are other tantalizing clues, though perhaps these connections strain the bonds of credibility. For example, is it possible that the Skaal deity, the All-Maker, is some distant echo of mighty Alduin, the World-Eater of the ancient Nord pantheon?
The book says that the All-Maker may be a distant echo of Alduin the World-Eater. To iterate, the All-Maker is a belevolent all encompassing being who created everything, while Alduin is the dark devourer of the world. The two are nothing alike. But it gets even more blatant, as the Skaal already have their own version of Alduin, who's an aspect of the Adversary, the enemy of the All-Maker and the Skaal.
In a time before now, long before now, when the
Skaal were new, there was peace in the Land. The sun was hot and the crops grew long, and the people were happy in the peace that the All-Maker provided. But, the Skaal grew complacent and lazy, and they took for granted the Lands and all the gifts the All-Maker had given them. They forgot, or chose not to remember, that the Adversary is always watching, and that he delights in tormenting the All-Maker and his chosen people. And so it was that the Adversary came to be among the Skaal.
The Adversary has many aspects. He appears in the unholy beasts and the incurable plague. At the End of Seasons, we will know him as Thartaag the World-Devourer. But in these ages he came to be known as the Greedy Man.
So despite Tharstaag being the World-Devourer for the Skaal, who eats the world at the End of Seasons, the Lucius Gallus still equates the All-Maker to Alduin (for the reason that all I can make out, because their names both start with 'A').
To put this further into context, there are other in-universe books which also claim to be authorative. There Be Dragons (written by Torhal Bjorik) says that dragons summoning storms and stopping time should be dismissed as myths and fairy tales.
The more fanciful tales have them summoning storms and even stopping time. These should be discounted as myths and faery tales.
Which is of course, completely wrong, as summoning storms and slowing time are powers we can directly see in the games themselves (which Dovahkiin can use).
So this is another source which says something is just a myth when it provably isn't.
So all this being said, take what you will from the story, but 'it didn't happen because the book itself says it's just fantasy' is not a valid argument.