I know there is great debate with respect to some of the things at The Imperial Library in terms of whether or not they're canon, but if one in particular is, it strongly suggests that the world has been created and destroyed many times over, and furthermore, that Mehrunes Dagon once attempted to save the world from being devoured by gradually hiding and re-adding bits of the world each time it was created, so that each world created was successively larger than the last. Eventually, he hoped that the world would be too large for Akatosh/Alduin to "eat." Akatosh/Alduin discovered this treachery, and banished Dagon to Oblivion, from whence he could only be exonerated if he managed to destroy every last excess bit of the world he had added. The covenant with St Alessia and the Dragon Fires prevented Dagon from reaching Nirn to do so, however.
I can only speculate that Mehrunes Dagon's attack on Tamriel may well have been - far from the desire simply to destroy for its own sake - an effort to eliminate this excess matter on Nirn (perhaps this excess is Tamriel itself?) in hopes of freeing himself.
If so, then Akatosh/Alduin's covenant with Alessia - far from the benevolent protection of his former prey, which never made sense to me anyway - was in actuality a device to ensure Dagon's captivity, and a way to preserve creation for the appointed time, before the time of devouring/destruction came again, and Martin becoming the Avatar of Akatosh was merely a means of containing Dagon yet again (with the added bonus of snuffing out the line of Dragonborn once and for all, opening the way to the time of devouring... which will begin in Skyrim.)
That's just a theory, though.
I can only speculate that Mehrunes Dagon's attack on Tamriel may well have been - far from the desire simply to destroy for its own sake - an effort to eliminate this excess matter on Nirn (perhaps this excess is Tamriel itself?) in hopes of freeing himself.
If so, then Akatosh/Alduin's covenant with Alessia - far from the benevolent protection of his former prey, which never made sense to me anyway - was in actuality a device to ensure Dagon's captivity, and a way to preserve creation for the appointed time, before the time of devouring/destruction came again, and Martin becoming the Avatar of Akatosh was merely a means of containing Dagon yet again (with the added bonus of snuffing out the line of Dragonborn once and for all, opening the way to the time of devouring... which will begin in Skyrim.)
That's just a theory, though.
That... actually makes a lot of sense. So, the theory is basically that Mehrunes Dagon is a good guy. Cool.