The Quest for Truth

What's the trick with the orcs in mission 3? I didn't do anything, they just showed up and started destroying everything. What was the plot twist?

If you complete the optional quest in the demonic dungeon in mission 2 and sacrifice the orcs, they will rebel, accuse the mages, and later come to beat them and everyone around them. But they only spawn when the quest is completed, as an additional feature.