The Telvanni got facerolled by the argonian invaders after the Red mountain shat all over the place.
"Circa
4E 6, the last remaining stronghold of the House was sacked by the Argonian invasion force and Great House Telvanni fell. It is documented that some survivors attempted to flee aboard
The Pride of Tel Vos, a ship which was later found wrecked along the coast of Skyrim, east of
Winterhold."
i don't know if this first popped up in skyrim, or one of the books. either way it's fucking stupid. i can just imagine centuries old, artifact bling wearing, daedra pet keeping wizards that can teleport and fly, cowering in fear of the
methhistsaphead argonian savages, trying to flee via boat.
my reinterpretation of that is that the argonians just killed off the non-magelord noble house telvanni members and a bunch of retainers/mercenaries while trying not to get too close to port telvannis and other death-raining-from-the-sky hotspots.