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.
Mephala’s Nest Entrance.
Mephala’s Nest Entrance Map Location.
On a plate, south from Boethiah’s Proving lorebook.
Exact location of the shard.