Opus Magnum is part of the Miller Kreyzl route tied to 
Wedding Crashers.
The ultimate goal is to recover the special book Lightbringer's Secret for Kreyzl. You can only pick up that book after Back in the Saddle.
This side quest carries over through the wedding. Finishing Wedding Crashers does not fail it.
Start Opus Magnum after you have finished Materia Prima and Forbidden Fruit first.
Goal of this quest
Opus Magnum begins automatically when you speak with Kreyzl at 
Lower Semine Mill at the end of Forbidden Fruit.
Kreyzl wants a precious book from Trosky. His plan is to attend the Semine wedding with a lady impressive enough that Chamberlain Ulrich will invite her to his chambers in the castle. Henry poses as her noble bodyguard. While Ulrich is busy with her, Henry would steal the book unseen.
Events at the wedding derail that plan, but you can still continue the quest. First you must gather everything Kreyzl needs.
Item & wedding preparation
Go to Troskowitz and buy a red dress for your companion from Tailor Bartoshek.
Tyrol Brocade Cotchardie is the best option. It costs 275.9 Groschen. You also need high Charisma clothes for yourself for the wedding, so this tailor trip is expensive.
There are 22 Charisma Spectacles in a nest at the 
Bench under Trosky interesting site, on a road northeast of Troskowitz, if you want a free boost.
Next, obtain Mintha Perfume. Innkeeper Betty in Troskowitz sells it for 499 Groschen.
You can steal it from her or from her bedroom chest after pickpocketing her key, ideally at night while she sleeps.
Or brew it yourself with Marigold, Mint, and Dandelion. Use the bench near the Apothecary's Herb Garden interesting site, which has plenty of Marigold for the recipe.
Dandelions are common around Troskowitz. Mint grows in gardens or is sold by Apothecary Emmerich.
The Mintha recipe is sold by Pavlena for 164.4 Groschen after you complete the Bad Blood side quest. The potions checklist lists the same recipe as 
Mintha Perfume.
Find a suitable girl
Visit the baths at Zhelejov Wagoners' Inn and talk to Dorothy.
None of the bathmaids want to accompany you because of Chamberlain Ulrich.
Dorothy suggests asking Enneleyn, a girl who recently passed through toward Nomads' Camp.
You can find Enneleyn resting against a cart just outside Nomads' Camp on the main road.
Speak to most of the nomads or the Voivode for an exact map marker.
Persuade her to join you, give her the dress, wine, and Mintha Perfume, and change into your fancy clothes. She then invites you to travel to the wedding together.
The quest carries through the wedding and continues further after Back in the Saddle.
If you aim to clear all quests, this is a good time to also run Blacksmith Radovan's wedding line: The Blacksmith's Son, The Jaunt, and The Hermit.
After the wedding
After Wedding Crashers, the quest continues once you gain access to Trosky Castle.
Go upstairs to the scribe's room and ask about the book you need.
He explains that someone named Nikvard borrowed it and never returned it.
If you need scribe access during For Whom the Bell Tolls, follow that main quest.
Focus on objectives such as Help the Chamberlain and Get to the Towers until you can reach him.
Ask for Nikvard
You can ask several people about Nikvard. The reliable lead is Stablehand Hersh, near the small gatehouse.
Hersh suspects you are a spy. To earn his trust, give him a sword: forged, bought from the blacksmith, or the wooden practice sword near the small coal hut.
He then tells you Nikvard's location outside the castle.
Finally, find the book
Once you can leave Trosky during Back in the Saddle, search the outer castle walls.
Nikvard's body lies near rocks by the large white tower. Next to him you find the book Kreyzl wants: Lightbringer's Secret.
Approach from outside the castle, roughly from where the three river streams are visible.
Climb until you find Nikvard's body in a lot of blood and dung.
Final book location:
Return the book and finale
Bring the book back to Kreyzl at Lower Semine Mill.
He sends you to The Lair interesting site, southeast of the mill across the bridge where you first met Bonnie.
If you saved the dog during Forbidden Fruit, he appears here too, along with Kreyzl and Zink, when they reveal their real plan.
Opus Magnum rewards
Instead of building a golem, they have been gathering ingredients for gunpowder.
With everything ready, they run the experiment. A short cutscene ends in a loud explosion and the quest completes right after.
The game awards the Lead Shot Gunpowder alchemy recipe and a large amount of XP across several skills.
Opus Magnum FAQ
How do I start Opus Magnum in KCD2?
Finish Materia Prima and Forbidden Fruit on Miller Kreyzl's route. Opus Magnum begins when you speak with Kreyzl at Lower Semine Mill at the end of Forbidden Fruit.
Does completing Wedding Crashers fail Opus Magnum?
No. The quest carries through the wedding. Finishing Wedding Crashers does not fail Opus Magnum.
When can I pick up Lightbringer's Secret?
Only after the Back in the Saddle main quest, when you can leave Trosky. Nikvard's body and the book are outside the castle walls near the large white tower.
Where do I find Nikvard for the scribe's book clue?
Give Stablehand Hersh a sword near the small gatehouse so he trusts you. He then tells you Nikvard is outside the castle.
What are the Opus Magnum quest rewards?
You get the Lead Shot Gunpowder alchemy recipe plus a large chunk of XP across several skills after the finale cutscene at The Lair.
How do I reach the scribe during For Whom the Bell Tolls?
Follow For Whom the Bell Tolls castle objectives. Use the steps Help the Chamberlain and Get to the Towers when they appear so you can reach the scribe's room upstairs.