Edge island

Hello everyone, I need help. I've destroyed all the enemies and captured all the cities except for the final one. I can't seem to reach it, even with boots of levitation. The only garrison left has thousands of dragons guarding it, along with a bunch of artifacts in the dungeon. But I can't figure out what artifact might help me. I don't see any scroll there, like the one mentioned earlier. What is this scroll, where is it, and how do I get to the final city?

Added 1 minute ago
This map is interesting; you play through it, and then suddenly you hit a dead end. How is that possible?