I'm sure they'll find a way to twist the Shadow to their advantage, and there are enough resources and artifacts there to conquer the world, and more than one.
As for Tartarrus, as I understood from the short scene, he wasn't respawned with max stats after all. Well, plus Karnee needs to ally with someone — you might be able to kill lords with assassins, but without a proper continental army, someone still has to hold the lands.
I remind you of Twice's fate, Heroist
why :(