Prison Cell Key

Prison Cell Key is used to open the prison doors that lead to Kharan Mighthammer, Marshal Windsor and Commander Gor'shak in Blackrock Depths. As of patch 4.0.3a the key still drops and opens the cells containing the NPCs; however doing so serves no purpose, as the quests involving these NPCs have been removed.

It was necessary for the completion of
 * Marshal Windsor (quest)
 * Jail Break!
 * Commander Gor'shak (quest)

Source
This key is dropped by High Interrogator Gerstahn in Blackrock Depths.

Note
Only one person may loot the key. Since it is unique, it would go to someone without the key.

The key used to appear in the player's Keyring, however this was also removed in Patch 4.2.0 so the key will now simply appear in the player's bag.