Guide:
Eastern Shadowfen, east of Alten Corimont, east of Alten Corimont Wayshrine, east of The Graceful Dominator.
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.
Couldn’t have just posted a map with a dot for the oath start?