Underground Inner Cell Chamber Key is a Key Item in dlc.
Bloodborne's The Old Hunters
Key to the inner chamber of the cell below the Grand Cathedral.
The innermost chamber of the underground cell holds a lone madman. He wears a beast hide, and rings a bell that emits no sound.
Unending death awaits those who can hear the soundless bell.
Opens the door to
Brador's cell in the Dungeon, past the Underground Corpse Pile.
Can be found on the floor if players killed Simon, or can be given to the player by him upon his dying breath.
One of the two keys that unlocks cells in the dungeon, the other is the Underground Cell Key.
The statement in the description of this Key, " Unending death awaits those who can hear the soundless bell.", is a reference to what Brador says upon killing the player: " Unending death awaits those who pry into the unknown."