Here’s how I completed this mission. At first, I was getting beaten up like crazy, but then I figured it out through trial and error: you need to rush towards the portal that leads to the underworld. Along the way, pick up everyone who joins you, and, of course, attack those who attack you, but don’t waste time on the orange patrol (they won’t attack you if you stay away from them). In the dungeon, run north; I didn’t bother with the purple faction, and I didn’t capture the castle. I emerged on the surface, right next to the blue castle (along the way, I reached level 10 and learned some spells), and without hesitation, I attacked the guy standing in front of the castle. He still had a weak army (and if you spend too much time on the map, you won’t be able to defeat him later). Then came the final cutscene, and we moved on to the next level!