Because Dagon (possibly) is Magnus.

That whole http://imperial-library.info/content/fight-one-eating-birth-dagon

'Mora may have re-purposed the Infium as it does fulfill the Blood of the Daedra quest in Oblivion, but no, http://www.imperial-library.info/content/morrowind-varieties-faith-empire It's not spelled out, but I think you get the picture. Although, Mora seems to imply based on your conversations with him that he provided the information to Xarxes. Judging by the convoluted nature of TES metaphysics though, I'd wager that Mora and Xarxes are really the same being and they just separate themselves for the sake of their followers.

Excuse me, that's Seven Fights of aLdudgjlgdshlablahblah.

Ah, I think I'm beginning to understand. So when Mankar said Dagon would claim his birthright he didn't mean that Mundus was the birtright, but that the invasion which would destroy Mundus would allow him to claim his birtright of being the kind Leaper Demon King. Correct?

Lorkhan, by M.C.'s estimation, is a Daedroth. As a Prince, he shares a 'lineage' with his 'siblings' - the other Princes of Misrule. This is where Dagon's birth right enters the mix. Dagon can claim his 'brother's' throne. The Commentaries say Lorkhan was disposedeposed, and the Magna Ge made Dagon, to free them from Aedric rule.

ignore this