Haven – "Everything is mixed up, truth and falsehood" [Campaign 3 – Mission 2]

My brother is standing in the other corner of the dungeon. And that entrance is in the lower right corner of the map, near the town. You go down there, walk a bit, and you come across closed gates; behind them is the city of the dead. You click on the gates, and a message appears: "Pass the trial of the dragon knights," but it's unclear what this trial is. I want to capture this city, and then the whole map will be mine. I've wandered everywhere, but there's no trial anywhere, and the gates won't open. And in front of my brother, there are no gates, only neutral armies. Help me with the trial; otherwise, I'm going crazy!!!