Sculpted by: Tony Cipriano
Release: 2020
Edition: Legacy
Used material(s): Cast in antique copper-plated white metal
One of: Unlimited

Signed: No

Issued in Ghost Key box

First appearance: Keys to the Kingdom #3
Allows the control of the Great Locke
The Great Lock

More information:
(possible spoilers)

The Chain Key was used to control The Great Lock, which serves as the guardian of the catacombs under Keyhouse. At one point, a group of criminals broke into Keyhouse Manor where they stumbled into the Great Lock, which incapacitated them and left them for Chamberlin Locke to interrogate.

On February 11, Dodge used the Great Lock to attack the Locke children who were able to neutralize it with the Chain Key.

Guide to the Known Keys:
“Something troublesome has happened. The Brougham Boys broke into the house this Sunday morning, while we were at church, and while attempting to enter the catacombs, triggered the Great Lock. I sorted them out with the Head Key, but I fear this is only the prelude to some new threat. The damnable thing is that they were sent with orders to break into the wine cellar, although they had no idea what they would find when they forced their way in. Someone must have some idea what I keep there, though, and someone is after it…”

Source: Locke & Key Wiki