Aurora Glade (mission)

At the end of this mission you find yourself at the Henge of Denravi. From there you get a quest to follow a spider (who soon dies), but who would have led you to a boatman. Give the boatman the correct answers, and when you are ready he will take you to Riverside Province where the mission is available.

Bonus
Rescue the ranger with the odd name from White Mantle attacking him. He will tell you of the Demagogue, a powerful member of the Mantle whom you should kill.

When you get to the final crystal stage (where you have to convert the 4 pedestals while the Mantle also send runners to do the same), the Demagogue is positioned near where all the Mantle are coming from. I think the best way of doing this is to kill all the Mantle's runners and drive your way to where the mantle are coming from, which means you shouldn't capture all the points until you are done.