Necromancer – “Invasion” [Campaign 3, Mission 3]

Here's how you should proceed: Mages appear at the very beginning of the mission, and you can't fight them (as per the storyline). When you get close to them, you need to run away and break away from the chase. The mages move along their route, so you need to wait for the right moment and run to the right, entering the dungeon. Of course, you need to collect all the undead on the map and fight the living monsters to increase the number of skeletons in your army. After you run past the mages and enter the dungeon, they will still follow you, so you can't hesitate and need to keep moving forward. You exit the dungeon and kill the hero near the castle, and that's it. The mission is over.