Grim Guzzler Key

The Grim Guzzler Key is a key that is carried by Plugger Spazzring, the bartender of Grim Guzzler in Blackrock Depths. This key opens the rear door of Grim Guzzler. This allows access to the deeper areas of Blackrock Depths.

Source
The key is carried by Plugger Spazzring. It can only be attained by pickpocketing him, and does not drop from his corpse.