Morrowind... OH WHY OH WHY DID THEY KILL MORROWIND!?
You know, I would actually be okay with Morrowind being destroyed if there was actually a good reason behind it. But no, the actual reason is completely stupid. Essentially after the defeat of Dagoth Ur, the Heart of Lorkhan is destroyed. This is where the Tribunal got all their powers from. So basically without their powers, they turn back into useless mortals. Or you could just kill the entire Tribunal because Morrowind actually let you kill everyone. I personally slaughtered all 3 of them. Anyway:
See, Vivec, although portrayed as caring and loving of his people, was actually kind of a dick. You know that giant rock hovering over his palace? The Ministry of Truth? Yeah, well, that rock was originally a meteor sent my Sheogorath to destroy Vvardenfell. Vivec stopped the rock of course, since he's basically Chimer Jesus at full power, but in order to ensure everyone kept worshipping him as a god, he kept the rock stationed above Vivec, so basically if he stopped being worshipped he'd take every single person with him. So because you either kill Vivec directly or destroy Vivec's source of godhood, he disappears one way or another. And when he disappears, the Dunmer have to build a machine to keep the meteor from falling and killing everyone. So they build a machine that runs off souls and soul gems. This works for a long time, 50 years or so, until one day they're about to sacrifice this woman. Well turns out this woman has a fiance. So the fiance shows up, BREAKS THE MACHINE, and KILLS EVERYONE WORKING ON IT, just so he can save his wife, everyone else be damned. So because this dumbass destroyed the only thing holding up the Ministry of Truth, it falls, killing him, his wife, and the entirety of Vivec City. The entire city is gone. The shockwaves from the meteor cause the Red Mountain, a still active volcano on Vvardenfell, to erupt. This eruption literally kills every single person in Vvardenfell, it's completely gone. Just wiped out. It also destroys a bunch of mainland Morrowind as well. When the Argonians from Black Marsh find out about this, they take the opportunity to invade and free all the enslaved Argonians and Khajiits working on Dunmer plantations, which wipes out a big chunk of Dunmer and sends a lot of them packing to Skyrim and Cyrodiil. In fact, most of the Dunmer you see in both Skyrim and Oblivion are actually refugees fleeing from the destruction of Morrowind.
So TL;DR: Vivec was an asshole and was holding a massive meteor over everyone's heads as leverage in case they decided to stop worshipping him and start worshipping the Daedra again. When he dies or disappears (player's choice), the meteor starts to fall. They build a soul machine to hold it up, but this one moron doesn't want his girlfriend to be sacrificed to the machine, so he saves her and this causes the meteor to fall, killing everyone, including the 2 dumbass fiances, and this leads to Red Mountain erupting, which wipes out both Vvardenfell and leads to an Argonian invasion that nearly destroys the rest of Morrowind.
Also cryrodil was said to be a jungle not General fantasy land #9986754302
Actually
@R.Graves , they explained that by essentially saying Talos (AKA Tiber Septim) used the power of the Thu'um to literally shout the jungle away. He used his shouts to blast all the jungle out of Cyrodiil. It sounds ridiculous but also kind of badass at the same time.