This is an unmissable achievement for completing Mission 6 - “For Whom the Bell Tolls” (Metallica reference?).
This worked for me -
- Carry the 3 sacks.
- Gain access to the horseshoe forge but there’s no need do the blacksmithing (I wasn’t very good at it).
- There’s a red bag on the back right shelf, rob the lockpick from it.
- Go up the stairs past the kitchen, right to the top to the alchemy room (trespassing area) and you’ll see a familiar face. Unlock the chest with the lockpick and take all ingredients. On the shelf in the corner is the alchemist book.
- Make the Fever potion.
- Go back down past the kitchen, to the opposite side and go through the door on the right (one goes up and one goes down), this is a trespassing area. Go up the stairs and you’ll see another guard blocking some more stairs. Use the rock to distract and go all the way to the top. Talk to the women there and give her the potion.
- After the cutscene, you’ll get the achievement.