In a hidden cave behind a waterfall underneath the Narcion node. I really messed up finding it, fell down into the water and couldn't figure how to get back up the cliffs after getting the knowledge
When you are trying to get down there choose the left side of the right bridge (i.e. east side of the more western bridge) and go down the cliffs. Additionally, you can use the hunter's clothes that give -50% fall damage. It made me survive the big drop even from the right side of the bridge.