Witcher 2, The - Assassins of Kings Message Board  

Try the forest that you can reach while going through where the Scoia'tael are present.
You have to be close to the Vergen gate to get to that place. After that go through a cave, until you reach a peaceful river. Some non-human mercs should be there to try and stop you.

Cross the river where it is shallow and go into the forest.

The forest is a bit tricky. Try looking for ledges you can climb up. You have to find a path on the map that is actually on a cliff.
It should be easily climbable and you only need to follow the path and you'll reach the tower with an elite harpy in it. There will be some other harpies present. The harpy nest with the crystal is accessible through a climbable ledge.