Necromancer – “Invasion” [Campaign 3, Mission 3]

Curse the mage cities. To do this, put on the cloak that remains after killing one of the mages. It seems to give -20% to enemy initiative, but I'm not sure. Enter each mage city and exit. You will be offered to curse it. Curse them, and a script will trigger, allowing you to enter the portal. I think that's how it works.