Islands of Mystery, Chapter 2: Dragon Wisdom

Or instant magical omniscience.

And here's another thing: if a mage has Vampirism, and it makes sense to use it on creatures with attack-retaliation, which ones should you target? Considering that 2 units are golems and gargoyles, you can't build titans in this city in this mission, and gremlins are also out. That leaves rakshasas, djinn, or mages.