Located inside the “Atanaz Ruins” solo dungeon. Start at Alten Corimont Wayshrine and head east. Swim across the river and you will come to a small peninsula, with a boat on its shore. Rock formations will prevent you from going to the top of the hill on the eastern part of the peninsula, so you will have to go north, around the hill and there you will see a small path leading south, uphill to the entrance of Atanaz Ruins. The skyshard is in the second square room, inside the southwest stone pillar.
View of the path coming from the north, leading to Atanaz Ruins.
Atanaz Ruins, the entrance is on the top.
Dungeon entrance location on the world map.