"Flight to Salvation" - "The Veil is Torn" [Campaign 4, Mission 2]

Please help, I haven't been able to figure out how to complete it for 3 days. (I move the city, defeat everyone, and go to the dungeon, but I keep hitting a dead end. There are obelisks, and the map has some kind of structure. I run there, and there's a crystal and a head, and in the middle, something that looks like a crystal too, but I can't take it. What should I do? Please provide detailed instructions. Thank you in advance.)