Quest giver: Steffen
Learn more of the grimoire
Explore the Thieves' Den
Deliver the Grimoire to Steffen
Talk to Steffen in Gran Soren to start the quest. Next, speak with Camellia in the apothercary shop in the Fountain Square to update the quest. Make your way to Ruins of Aernst Castle, west of Witchwood. There are hostile bandits outside the area though the ones inside the ruins will remain neutral and will react only if you attacked them.
Climb the topmost portion of the main tower to find the grimoire at the top. Deliver it to Steffen to complete the quest.
Note: Completing this quest will allow Steffen to help you in a quest later in the game.