The Truth is in the Stars is a Side Quest in Velen.

Head to the town of Benek, east of the Reardon Manor and Downwarren. Go right of the big windmill and you will see an old man sitting outside his cottage smoking. Talk to him and he will tell you your future in exchange for some food.

After you give him food he will tell you your future. However, the events already happened and his gift isn't as strong as it used to be, because of his age. He will then task you to find a dragonsroot to help him see your future. Travel southeast to a cave, on your map its where there is a small path the branches off from the main path.

Once in the cave you will find four or five level 2 ghouls. After you killed the ghouls grab the dragonsroot that is right of a dead body and up against the cave wall. Head back to the old man and he will tell you your future.