once you are there follow the path with the alien looking creaters until you reach the large pool of river.
Cross the two trees and attach to the left handside of the wall until you enter this mini cave .
The Swarm of Flies is located at Mohgwyn Palace, the underground region near Siofra River. Travel north of Palace Approach Ledge Road grace, you'll find it in a small cave to the east. and i would suggest you to go over this once