Inside the “The Corpse Garden” public dungeon. Start at Eidolon’s Hollow Wayshrine and follow the road going west. When you pass Shrine of the Saint Veloth leave the road and head north. After a while you will see the entrance into the dungeon. Enter the dungeon, follow the path and in the southern hall you will see the skyshard standing in the middle of the circle filled with sand.
The Corpse Garden entrance
The Corpse Garden shown on the map
Shard is found in the south room inside a circle filled with sand.
Dungeon map showing the exact location.
Sand Kingdom Moons - Super Mario Odyssey Power Moons