This achievement is connected to Undead princess storyline.
*** Spoiler - click to reveal ***Her story will branch after testing Imp mixture on orcs/undead. If you want to get both achievements for her you need to pick option on using prisoners to return testers souls to lives. Then you get option to search for the most beautiful maiden in the realm. You need to pick this for both achievements. When you have said maiden SAVE because here the story is branching again. For this achievement you need to sacrifice the maiden. After succesful operation the princess will have doubts about her mortality and will ask your opinion about making the deal with the demon. To get this achievement you need to encourage her to make the deal.