Kreyzl needs you to track down a special alchemy book that he can't find anywhere else, and you need to sneak into the castle to find it.

At this point, and if you pushed through the Opus Magnum side quest, you’ll have the opportunity to track down the book for the miller. Trosky Castle is large and has a confusing layout ...

The Opus Magnum side quest in Kingdom Come Deliverance 2 is a continuation of the chaos that started with Forbidden Fruit.