Ideas for a revival.

The Swamp Dwellers were the only faction that supported the Barbarians. And after Kilgor and Gelu clashed swords and an explosion occurred – a very powerful one, but not completely destroying the planet – the hatred of the surviving humans, mages, elves, and others turned against the Swamp Dwellers (since almost all the Barbarians had perished). That's why the Swamp Dwellers had to retreat. They fought their way across Erathia and were eventually forced to go underground. Something like that.
P.S. – Did you watch the first campaign?