The Witcher 3: The Truth is in the Stars

You’ll find this quest in Benek. If you haven’t unlocked the signpost for Benek, go to Downwarren, then head northeast.

Outside the house you’ll find an old man who you can play a game of Gwent with, and then offer him some food.

After this encounter, head over to the cave, where you’ll have to kill a whole bunch of Ghouls. Now just search using your Witcher sense for Dragonroot and take it back to the old fella to complete the quest.