User blog comment:Roxashasnoheart/Who would win? Alduin or Mehrunes Dagon?/@comment-108.211.252.76-20150623210846

Neither would win because neither can truly die, but ultimately, a deathmatch between Alduin and Dagon is impossible because of the barrier separating Oblivion and Mundus.

However, if Alduin had been sent forward in time to final moments of the Oblivion Crisis when Mehrunes Dagon entered the Imperial City from Oblivion, then I suspect Alduin might be able to banish Dagon back to Oblivion, but just as the Avatar of Akatosh itself was mortally wounded by Mehrunes Dagon and turned to stone after the battle, it is reasonable to assume that Dagon would inflict enough damage on Alduin to banish him from Nirn as well.

It is important to note that Alduin, although firstborn and an aspect of Akatosh, is not actually a god in the sense that Mehrunes Dagon is. Both Alduin and Mehrunes Dagon are not their most powerful in Mundus, Dagon being bound to the Oblivion plane of the Deadlands and Alduin's height of power being in Sovngarde (where he was defeated).

Alduin exists at the beck and call of Akatosh, who is powerful enough to hold back the forces of Oblivion and the Daedric Princes in their true forms, but the Aedra are actually weaker than the Daedra, being the et'Ada who gave a portion of themselves in the creation of Mundus, being bound to Nirn and unable to interact directly with the world.

In terms of blatant power, as a Daedric Prince, Mehrunes Dagon is undeniably more powerful than Alduin and even Akatosh, but this is mostly within his Oblivion plane of the Deadlands, and it is actually suggested that a Daedric Lord is one with their plane, and in order to truly destroy a Daedra Prince, one would essentially have to destroy their plane of Oblivion.