>I remember reading that this kalpa is the dream of a major being from the previous kalpa. I forgot who/which it was… Anui-El? I'm probably mixing things up, though.
Not quite. Someone from an entirely previous universe, not just a previous kalpa in this one. It was Anu, a being who projected himself as the concept of Stasis in this one. Anui-el is his soul, so you pretty much got that part right, mostly.
>If a kalpa ends, what gets destroyed and what remains? Is only Mundus destroyed, but Aetherius remains (since IIRC the Redguards wait out the transition at the Far Shores, which is part of Aetherius)?
Mundus is destroyed, but at least part of Oblivion remains, because the Hist hid in Oblivion. Aetherius is outside of even that, and the Yokudans hid out there, so presumably it remains too, yeah.
>wat
Just a theory a gay theory
Assuming he's not referring to the erroneous notion that breaking all of the Towers will end reality (it will only allow Tamriel to sink into the ocean, like Lyg and Yokuda before it) or the mistaken idea that C0DA is an inevitability, he's probably talking about the ending of Skyrim.
Paarthurnax tries to talk you out of going after Alduin, saying that sometimes something must end to make way for something new and so on (he even namedrops kalpas). It's fairly well-known that Alduin eats the world so that a new kalpa can begin, resetting everything before things go too far off the rails and something that DOES threaten all of reality happens. This kalpa, Alduin was already being a bit weird, and for that he was sealed away until a future date, when he would be unleashed to do what he was meant to. But instead, the Last Dragonborn kills him, preventing the kalpa from ending.
Of course, you don't absorb his soul, he just breaks, which, while he breaks pretty badly, is still probably something the Jills will fix as well as they can. The Greybeards pretty much tell you that means he'll be back one day.