Guide:
Northeastern Rift, north of Riften Wayshrine, or southeast of Fallowstone Wayshrine. Inside the Fort Greenwall public dungeon.
Start in Riften and follow the road going north. After a while, you will arrive at the closed southern gate. You’ll have to go around, to the east, following the fort wall. There is a crumbled part of the wall on its eastern side where you have to climb up and enter the fort courtyard. The entrance to the dungeon is just straight across that point, to the west.
Once you enter the dungeon and approach the first intersection, go into the hallway to your right. At the center of this hallway is a giant hole. Jumping into it, you’ll enter the lower level of the dungeon and the exact location of the shard.
Crumbled wall on the eastern side of the fort. Be careful when you climb up, there is a group of NPCs right above.
You can just go in the gate. You don’t have to find the broken wall.