Start at Coiled Path Landing, and go south. You will reach a river going outside of the map. Just follow it until you see a small island in the middle of the river. The skyshard is located on this small island, in a wooden crate, next to a tree and a big stone. The “Ebony Blade History” lorebook is next to the wooden crate too.
Following the river will get you close to the small island and the collectible on it.
The location on the world map.
Ebony Blade History and the skyshard next to each other.
Sand Kingdom Moons - Super Mario Odyssey Power Moons