Board Thread:Oblivion/@comment-98.231.109.104-20140510071351/@comment-5262613-20140518220931

well we know now that there was indeed an invasion during the Interregnum of the 2E, what with ESO's release. but the Oblivion Crisis was bought about due to Mankar Camoran and his Mythic Dawn cult acting as agents of Dagon. the Daedra cannot invade Tamriel without mortals to open portals to one of the many realms of Oblivion. so i guess that not many people were a part of Daedric cults hellbent on destroying Tamriel during the Interregnum (aside from Mannimarco acting on behalf of Molag Bal). If Dagon could have crossed over without help from mortals, he sure as oblivion would have done it. this is definitely not the best explanation you will get as i am positive others are much more in tune with the lore than i am, but i hope it gives you somewhat of an answer