In "For Whom the Bell Tolls," a main quest in Kingdom Come: Deliverance 2, Hans requires your assistance at Trosky Castle. This involves crafting a Fever Tonic using alchemy.

Locating the Fever Tonic Recipe:

The recipe resides within the Surgeon's Workshop in Trosky Castle. Access is gained either through stealth or persuasion. To persuade the chamberlain, complete the "For Whom the Bell Tolls" objectives up to delivering the horseshoe. Then, speak with Cook Fanka at the forge, offering assistance with her locked chest. This leads you to the noble's kitchen and the ailing chamberlain. Successfully completing skill checks convinces him to grant you workshop access. Alternatively, if your skill checks fail, steal food, proceed to the other courtyard stairs, and bribe the guard with the stolen food to access the kitchen and sneak to the workshop.

The Surgeon's journal, containing the recipe, is on the bookshelf.

Brewing the Fever Tonic:

The necessary herbs are already at the surgeon's station. Follow these steps:

1. Pour wine into the cauldron.
2. Add 3 handfuls of Feverfew.
3. Lower the heat and boil for 2 turns of the sandglass.
4. Grind a handful of Elderberry Leaves.
5. Add the ground Elderberry Leaves.
6. Add 2 handfuls of Ginger.
7. Obtain a phial from the bottom-left shelf.
8. Distill the tonic using the Still.

Additional herbs are available in the chest if needed.

Completing the Quest:

After brewing the Fever Tonic, locate Thomas. If you gained the chamberlain's trust, return with both the Fever Tonic and a Digestive Potion. Otherwise, use the kitchen to reach the chapel and find Thomas and Adele. Speak with Adele to conclude the quest.