On this page of the Baldurs Gate 3 guide, you will find information to help you successfully solve the moon puzzle.
During the Find the Nightsong quest, you'll have to solve a puzzle. There will be 4 panels are showing the phases of the moon. To open the passage to the caves leading to the Underdark, you have to set them that a full moon is at the top, a new moon at the bottom (highlighted circle), and the crescents on the sides.
The correct solution requires moving the panels as follows:
- 2 x top
- 1 x right
- 3 x top
- 3 x left
- 3 x bottom
- 2 x left
The final layout of the panels should look like this: