First appearance: Crown of Shadows #3
Use: Control shadows
Door: Chamber of Living Shadows and the Crown of Shadows
Dodge found The Shadow Key in the basement in issue 3 of “Crown of Shadows” inside a glass of jam with “Black Currents” written on it (a pun by Rendell to hint at its location).
The shadow key opens a door in the basement which leads to the “Chamber of Living Shadows,” a room which contains candles and the Crown of Shadows on a stone pillar. Once the key is used on the door, the key starts to glow like a candle (it does not appear to generate heat however, as Dodge still used matches to light the candles inside the cave). By putting the key on the crown it is activated. If the user then puts it on his head, all shadows around him including his own come to life in different shapes. It is unknown how the shadows gain the form they take, but it appears that the user’s shadow gives hint to his personality (for example Dodge having a wolf and Tyler, with Rendell, having centurions). If the user is skilled with the usage of the Crown, he can “wear” shadows and take on a certain shadow-like appearance. He can also merge with also the shadows to become a gigantic shadow entity, increasing his height, strength and stamina.
Wearing The Shadow Crown gives control of the shadows in the Cave of Living Shadows. The shadows may some times be reluctant, but are always faithful to the wearer.
The shadows brought to life by the crown seem to be unique to each person.
Source: Locke & Key Wikia