In Skyrim, there are a lot of enemies, but the majority of them are nameless. The average person could probably name Alduin as a bad guy, but Miraak and Harkon are the other two main bad guys, and you… don’t really see much of them, let alone see them together in one place fighting to the death. But who would win in a fight, Miraak or Harkon?
Let’s size our two combatants up first.
Miraak is known as the First Dragonborn. He could kill dragons before your Dragonborn’s great, great, great, great, great grandfather was a twinkle in his great, great, great, great, great, great grandfather’s eye. That’s how old Miraak is. But ever since he tried to claim the lands of Solstheim for himself and attempted to defy the dragons who ruled at the time, he’s been stuck in Apocrypha, the Elder Scrolls’ equivalent of a Lovecraft fan fiction. Miraak can bend the will of dragons and absorb their life to reclaim his health, as well as use a handful of dragon shouts.
Harkon is the leader of the Volkihar vampire clan, who got his vampirism from the King of Domination himself, Molag Bal. He’s not as old as Miraak, but Harkon is still older than Talos’s Empire. Harkon has your standard vampiric abilities – resistance to cold, weakness to fire, the ability to turn into a bat and heal by sucking the life out of you. He’s also got a sword that heals him and the ability to float in mid air.
When it comes to lore, Miraak has the upper hand, since he has two deities behind him – Hermaeus Mora and (technically) Akatosh, while Harkon only has the backing of Molag Bal. Both are technically immortal and can’t die by normal means, but Harkon is really just a powerful but otherwise standard vampire. Both have their own sets of minions, but Miraak’s will-bent dragons would kick the shit out of the entirety of Harkon’s minions – both his vampire buddies and his skeletons and gargoyles, simply because most dragons breathe fire and vampires have a weakness to anything vaguely warm.
That’s not to say that both are weaklings though. While Miraak is strong, he is still confined to Apocrypha and has a limited supply of dragons, while Harkon can leave and attack anywhere during the night, and has the power and riches to send people out to do his bidding for him. In a one on one fight though, Miraak has the upper hand no matter what as he can only be truly killed by a Dragonborn or a Daedric Prince.
Now, in-game, the fight is actually a little bit awkward. From the point of view of their fights against the player, both Harkon and Miraak have rather scripted battles. Bug-wise, Miraak is absolutely insane and it’s required to make a save just before the final battle against him, just in case he glitches out and becomes invulnerable. Harkon though isn’t much better and his fight is more tedious than difficult, especially if you give him Auriel’s Bow before the fight starts, at which point it will go on forever as you can’t reliably break his shields. And assuming that Harkon doesn’t become invincible when he’s nearly dead and cause you to start over. Either way, both their fights are dangerous and buggy, but you can tell earlier on when Miraak’s is broken, as he tries to absorb his dragons.
Once again though, Miraak does have the upper hand off the bat, having a maximum level of 150, compared to Harkon’s 60. But Harkon has much more invulnerability than Miraak has. Miraak has Become Ethereal, which doesn’t last nearly as long as Harkon’s Mist Form. Harkon is also capable of recovering stamina and magicka, while Miraak lacks the Feim perk which allows him to regenerate anything at all. Unfortunately, a fight between them becomes one of attrition, which isn’t very interesting. Still, the fact that Harkon has a 50% weakness to fire, and Miraak can use fire-based attacks continues to make things easier for Miraak.
In the end, I finally managed to get these bastards to fight each other in-game. It required a lot of cheating and using console commands and the like, as well as constantly trying to stop them from aggro-ing on me, which was my major problem. Turns out, you put these guys in a room together, they just stand around and look at each other. They also don’t attack you by default until you cast a spell on them.
I eventually managed to get them to fight, and just as I predicted, it wasn’t even a contest. Miraak used his Firebreath and killed Harkon with relative ease, only suffering minor injuries. Both during the day and during the night, since Harkon doesn’t have extra fire weakness during the night. Harkon seemed unable or unwilling to use his ranged attacks, and running face-first into a Dragon Aspect-using, fire-breathing Miraak is never a good idea.
So, really, no matter how you look at it, Miraak would win.
But the real winner here is Alduin, who, lore-wise, is capable of literally eating the world. Alduin may seem weak in-game, but that’s because he’s only a ghost of his former self, and the Last Dragonborn makes sure that Alduin goes back to his proper job of ending universes.