There are two shards at this location. This one is not inside the solo dungeon. You can find it standing next to a small stone wall next to the path that leads to Fort Sphinxmoth.
Close to Fort Sphinxmoth Wayshrine you can find Rafora Casca. Rafora gives the quest “An Affront to Mara” – part of Fort Sphinxmoth Objective. This quest takes you to Marcella Ammianus and entrance to Fort Sphixmoth. They are both close to this collectible.