Can someone please help. I have finished off the Serpent's Temple except I can not find the key to free the prisoner. Does anyone know where I can find this key

Thanks

The Inquisitor dropped the key when I killed him. I have heard you can also get him to give it to you by talking. I'm fairly sure NPCs don't always drop the items they're supposed to. I killed Krueg twice (different saves of course) and he once dropped the serpent key, and once didn't.