Sculpted by: Jamie Macfarlane and Israel Skelton
Release: 2013
Edition: Limited
Used material(s): Cast in Roman gold-plated pewter
One of: 1000

Signed: No

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