A lost Elderling city, Frengong was buried in some sort of ancient disaster. Its ruins are the foundation of Trehaug, the city of the Rain Wild Traders. The Rain Wilders search the city for ancient treasures which they sell. The ruins are awe-inspiring (due to the glimpses they give of the Elderlings' civilization) but also threatening (due to the straightforward dangers of earthquakes, and the more complex dangers of dabbling in another civilization's magic).
The trade in magical goods has made the Rain Wilders wealthy, but the dangers that accompany it leave them vulnerable.