We have marked the location where this puzzle starts in a screenshot down below. As its name suggests, it will take place underground. When you get here, you will notice a picture of a sun on the wall. The sun is surrounded by several stars, three of which are black, and two which are white. This is the game’s way of giving you hints about what you need to do here. The solution to this puzzle is to light the three black stars. So, turn around from the picture, facing the chest and torches in the front. Now, select a Pyro-based character. Our go-to choice for whenever we have to light or explode something like this is Fischl. As a bow-wielder, this means that she can stand in one place and shoot without needing to move.
Regardless of which Pyro character you choose, you will need to hit and light up three torches. While facing the chest in front and the picture of the sun at your back these are: the torch to the right of the chest, and the two torches directly behind the chest. Once these three torches are correctly lit up, all other torches will also ignite, and you will be able to loot the chest for your rewards.