Walkthrough for the "Tribes of the East" expansion.

I'm stuck on the third zone of "Wisdom of the Earth." I've killed everyone I could and collected everything I needed. I went to the yellow tent, bought the machines, and visited the area behind the blue barrier, but the sorceress won't let me pass, and a battle zone opens up. Yes, the sorceress in the hut mentioned a portal nearby with an artifact, but I haven't found any portal. Could that be the reason I can't proceed? If so, where is this portal? I've searched everywhere, but I can't find it.