The Shadow Wars endgame consists 20 Sieges where you'll be tasked with defending your fortresses against invaders multiple times, with each invading force being tougher than the last. The main story may be over, then, but that's not the end of the game - the completion of your task and the true ending to the game now await you, but getting there is going to take a lot of work.
Watch on YouTube The Shadow Wars endgame explained - how to unlock Shadow of War's true ending