what to put in ancient altar bg3

To solve the Ancient Altar puzzle in Baldurs Gate 3, you need to place an item on the altar. The Ancient Altar is located in Balthazars room in Moonrise Towers, and you can access it by interacting with a secret book on the bookshelf. The item you need to place on the altar is a heart, which can be found on one of the bodies in the room. Once you have the heart, simply place it on the Ancient Altar to solve the puzzle and open a hidden passageway behind the bookshelf.