THIS IS A MISSABLE ACHIEVEMENTRequires you to make a certain choice near the end of the game. I will put the solution in spoilers but it will reveal very little so it should be safe.
*** Spoiler - click to reveal ***When you get the option to drink from the well make sure you choose the inquisitor and not the other person. After I believe another short main quest you will have to get a special ally. Once you do that you should get the achievement.
*If you are going to dislike my solution give me some criticism so I know what to fix*