It's simple, I have the latest patches installed.
You need to enter the portal with exactly 20 Bone Dragons, no more, no less.
You can get Bone Dragons by taking Colossi from the mages and transforming them into Bone Dragons using a building in the Necropolis. Then (I'll tell you how it worked for me): a mage named Maahir attacks me. I traditionally beat him up, build a building for transforming units, transform the Colossi into Bone Dragons, and jump into the portal to find Cyrus. That's it, you've completed the mission.