Inside the Balamath dungeon, southeastern Craglorn. North-northeast of Elinhir Wayshrine, northwest of Trial: Aetherian Archive, south-southeast of Inazzur’s Hold Wayshrine.
Start at Elinhir Wayshrine and head north until you reach the Magical Anomaly crystal. There is a short, narrow, passage across the river leading to the dungeon entrance. Skyshard is in an open area, just as you get out of the hallway. There are benches on the left and a vault entrance to the right, with the skyshard located above it.
Dungeon entrance location.
You can approach it either straight through the courtyard or through the vault filled with mobs.
The skyshard location on the dungeon map.
Sand Kingdom Moons - Super Mario Odyssey Power Moons