Key to the barred window in the Irithyll dungeon.
But the window only leads to a bottomless black pit below, and above, a paltry view of the ruined Profaned Capital.
The very architecture appears to be a cruel joke the jailers played on would-be escapees.
Used to open a shortcut from the beginning of Irythill Dungeon
Found in a cell in the hallway that is accessed by traveling through the holes in adjacent cells