The introduction of the rulebook of Betrayal of the Second Era (which may still be a WIP, so keep that in mind), says that Tamriel is a nexus of magical potential, and its history (past, present and future) is written and rewritten in the Elder Scrolls, prophetic texts that exist outside the normal confines of reality. Tamriel has only just survived a mysterious outbreak of the Knahaten Flu, and narrowly beat back an invasion from Akavir. While the Three Banners War rages, the Order of the Black Worm is using Dark Anchors to bind Nirn to the hellish planes of Oblivion (in particular the planes of Coldharbour).