Guide:
Central part of Stonefalls, inside Mephala’s Nest – public dungeon (torch icon), head to the east from Brothers of Strife Wayshrine.
In order to enter Mephala’s Nest, you have to find a cellar door that leads into it. The door is hidden on the round, stone platform. As you enter the dungeon, turn right and keep going in that direction. Inside the northernmost room is where you can find the shard. It is in the center of the room, next to a couple of enemies.
There is no road between Emberflint Mine and Mephala’s Nest, as the in-game map indicates. You will have to walk through the Mountains North of these sites, and enter Mephala’s Nest from the North.