Inside the “Gandranen Ruins” public dungeon. Start at Hissmir Wayshrine and head directly north. After you cross a river and a road you will come across the dungeon ruins. Stone doors leading into the dungeon are on the northern side. When you enter you will encounter a quest giver. The skyshard is in the hall behind her, to the left.
Ruins in the swamp.
Gandranen Ruins entrance
Questgiver name is Zahra, she is found right as you enter the dungeon.
Location on the dungeon map.