Observe the shadow on the sundial Genshin Impact's mysterious quest is solved. This quest is called Time and the Wind, and it involves heading to the north-east seas off of Mondstadt.
To go across the sea, you will need a whole lot of stamina to traverse the waters. You will want to whip up some food that grants stamina in order to make the trip. Alternatively, you can use Cryo-based characters to traverse the water more easily. Once you get to this island, the fun begins.
Observe the Shadow on the Sundial Genshin Impact
When you reach the island, start breaking open rocks. One of them drops a Ragged Notebook. The instructions it gives you are to observe the shadow on the sundial. Interact with the ravaged carving, and you'll want it to be 2:00 to 5:00 a.m in-game. This is so that you can use your Elemental Sight with a Wing character. This will reveal a path to a hidden Anemo element. Use your Anemo skill to now to get rid of the invisible wind cluster.
With the cluster now disappeared, deposit the wind in the middle of the island by casting the Anemo on it. This triggers a boss fight which we won't spoil, and then another area by which you should follow the 2:00 - 5:00 a.m. time frame interaction window again by making use of Elemental Sight.