This is part of the main quest and can't be missed.
After gathering the tablet and oil from two previous quests, head back to the Abbey of St. Hiram.
After a brief dialogue with one of the monks, head straight back and go through either door in the main room. Once through the door go through the middle door in the next room and approach the ceremonial altar. You'll be able to place the tablet and light the oil. Once these two steps are finished the achievement will pop.