Pathfinder - Wrath of the Righteous; A Kingmaker follow-up that supports turn-based gameplay now!

Who do you use midnight bolts on? I have 3 of them in my inventory. Its the major demon lords right? Not their underlings? You can only use 1 bolt per fight, right? How is this mechanic discovered normally? Are you expected to read every book and note you find and put it all together?