The Truth is in the Stars Walkthrough
Start Location: This quest can be acquired via a notice posted on the Oxenfurt Notice board.
Once you have acquired the notice, you’ll be able to start the quest by heading over to the small town of Benek on the eastern side of the Velen map. Here you’ll need to find and speak to the Old Sage sitting by one of the buildings.
During the conversation, offer him some food. He’ll then ask you to go and find him some Dragonsroot. This can be found in a cave nearby, so you may as well!
Follow the road out of town to the south and enter the marked cave. Make your way to the innermost part of the cave and clear out the Ghouls inhabiting the area. Bang on your Witcher Sense to find and acquire the Dragonsroot.
Return to the Old Sage and hand over the Dragonsroot for a scene to end the side-quest.