There was a civil war between the dragons a long long time ago with the winners imprisoning the losers.
Fast forward to today and very large, very random group of NPCs found out about this, freed one of the dragons that was conveniently kept separate from the others.
Also Sabellion > Wrathion and has been around longer, don’t be a simp.
While I agree with you, I also completely understand the OP not remembering a minor character from Burning Crusade who hasn’t been heard from in 15+ years.
razageth was as far as i can tell from the cinematics before df launched, imprisoned by getting tentacled by neltharion which likely explains why she was kept separate, didn’t want to risk the other dragons finding out via her blabbing about how it happened, or sensing some tentacly energy nearby.
Sabellian isn’t new, he was part of the Blade’s Edge Mountains quests in the Burning Crusade. How he came to be purified of Old God corruption is poorly explained, however (probably by writer’s intent).