Guide:
Northwestern Rift, northwest of Northwind Mine Wayshrine.
Inside the Snapleg Cave public dungeon. Start at Northwidn Mine Wayshrine and follow the main road going west until you reach a river, then hug the eastern river bank and head north. The entrance to the dungeon is where the river goes off the map, represented by a wooden door in a cliff.
In order to grab the shard, you have to find an open part of the cave. This part of the dungeon is found in the southeast. Once you reach this section, you should notice the shard. It is on a tiny hill, next to the large tree.
