Start at Venomous Fens Wayshrine, and go southwest. You can see a road going southwest, with a looping path on the northern part of it. Follow the road going southeast and it will lead you to a ruined tower. It’s a brown circle on the map, northwest of the tower icon representing White Rose Prison. The skyshard can be found inside that tower, on the second floor, in a wooden box.
Sand Kingdom Moons - Super Mario Odyssey Power Moons