Unicorn Empire

Hello everyone. I have a question: how do you kill Death Harbingers in mission 4? To kill them effectively, you need either Mummies or Death Knights, but I don't have them. Let me explain further. On my map, instead of Death Knights, Death Harbingers appeared, just like on the "Pack Leader" map. They have 300k HP and 1500 attack. But in the "Pack Leader" map, they are part of the scenario, and you are given Death Knights to kill them. Now, they have appeared on other maps, and I don't know if they are part of the scenario or just a bug.