This will give you 4 Legendary Wishing Pieces, known as Waterdude's Wishing Pieces.

These will instantly spawn a random Legendary at the Den, for you to either fight with NPCs or up to 3 other players. (Empty slots get filled with NPCs)

If you take too long before challenging the Pokemon after using the Piece, the Pokemon can despawn. 

These are also very difficult fights, so we suggest you bring allies. The NPCs may not be very good at fighting the Boss that spawns. 

If you fail the fight or take too long, the boss will despawn and you won't get your piece back.