In order to grab this skill point piece you need to enter Pothole Cavern – public dungeon. There is a camp of Shadowed Path enemies defending its entrance, as well as cave inside. Group of 2 mobs are regular. Patrolling creatures sometimes join this groups. Skyshard you are looking for is inside the largest Pothole Cavern room. It is on top of a small stone shrine, next to candles. This is a possible spawn area of Serrin Vol. He is a pretty tough mob to solo. In order to defeat him you need to annul his powerful instant spell. This spell is devastating and you need to find a way to survive it. He also spawns an add that helps him in combat. Another prize for defeating Serrin Vol comes in a shape of lorebook “On the Detachment of the Sheath”. This lorebook is in the same area where you can collect the skyshard.
Camp outside Pothole Cavern.
Pothole Cavern entrance map location.
Broader view on Pothole Cavern entrance map
Sand Kingdom Moons - Super Mario Odyssey Power Moons