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After two attempts, I collected the King of the Necropolis artifact and leveled up my Necromancer. I established myself in the right part of the map, gathered Liches, and started clearing out neutral creatures and weak blue heroes. The main hero went down and didn't come out. After that, I stayed in the castle, but I didn't have enough strength to break through the garrison. I also didn't understand where to go next. In the Dungeon, everything is clear, I know where to go. But on the surface, it's not. The main thing is to act quickly on the map and gather something useful.