The Opus Magnum quest is one you can get in Kingdom Come: Deliverance 2 if you’re willing to do work for Kreyzl, the miller. He’s one of the two tradesmen you can work for at the beginning of the game, and this request has you tracking down a particular book for him.
The book you need to find in Opus Magnum is an alchemy book from Prague. Kreyzl needs it to learn more about his experiments. However, it’s in Trosky Castle, a heavily fortified location that you can’t simply walk into and start searching for the book. Instead, you’ll need to gain access to the castle, and Kreyzl has a few ideas on how you can do this. There’s a more straightforward method than what he suggests, though. Here’s what you need to know about how to complete Opus Magnum in Kingdom Come: Deliverance 2.