If you look at the map, south of Russafeld there is a set of three small ponds, near the north part of the mountain. You’ll have to climb on top of this tall plateau. Once you’ve succeeded, you’ll spot the tall waterfall powering these small ponds. The very first one hides the skyshard. Although this is a pretty well hidden place, as you get close to it, you can’t really miss the light from the shard that’s lighting up the area. There is a ruined house nearby, but the best marker you can use to guide you is the waterfall. Just keep an eye on it, and you’ll find that 1/3 of the skill point right there.
Sand Kingdom Moons - Super Mario Odyssey Power Moons