Sculpted by: Arlen Pelletier
Release: 2020
Edition: Legacy
Used material(s):Cast in antique gold-plated solid metal
One of: Unlimited

Signed: No

Issued in Ghost Door box

First appearance: Keys to the Kingdom #3
Controls squirrels

More information:
(possible spoilers)

The Undertree Key was used against the Locke children during February.

Seems to allows the user to control squirrels.

Source: Locke & Key Wikia