I tried to look this up online, all I could find was that maybe he led a bunch of people to Outland and without a source of magic at their disposal a bunch of them died.
Other than that, I don’t really understand why he’s a raid (or dungeon, can’t remember…).
Christ, links on this forum are a pain. Google ‘wowpedia Kael’thas’. He joined the Legion, basically.
Dude went from “I will NEVER be desperate enough to side with demons!” to “Well, if I don’t follow Vashj to side with the demon Illidan, my people will all die…” to “I will serve you with enthusiasm and all of my heart, Lord Illidan!” to “Illidan isn’t doing enough for my people. I’ve already pledged myself to one demon, what’s a bigger demon?” and jumped ship over to Kil’jaeden. And from there… well, “THE WORLD WILL BURNNNNN”
Ah yeah, somehow I read something a little misleading about him and didn’t manage to see the WoW wiki page on him which makes it pretty clear what he did…