Ooh, the lore in Elder Scrolls goes way deep. I mean, there are more exotic concepts most know which are not really discussed in games like Lunar Lattice, Godhead and CHIM, but if you dig deeper, you're in for a real ride. Most of it goes way over my head, but concepts like time travel and AI from the future, Amaranth, and lastly, the Elder Scroll themselves about which Septimus Signus made simplified, but not inaccurate, in-game comments, are much discussed topics among the deep lore fandom. Plus all the more mundane stuff behind the scenes--dreughs, ayleids, Akavir and it's many inhabitants, sloads, Hist and their origin (discussed in Infernal City/Lord of Souls, but more questions raised than answers given. Hint: the Sleeping Tree is related to Hist and the rumor Ysolda dismisses as "nonsense" is literally true.) etc.

You could write a heavy tome about the history and deeds of the Dunmer people alone, let alone all the other events that happen(ed) in that universe.