First appearance: Keys to the Kingdom #3
Use: Controls owl automaton
Door: Automaton Owl
The Owl Key is used when Mark Cho finds Ellie unconscious in the Wellhouse, he takes the Owl out of his backpack and sends it to find Rendell. When it brings him, Erin and Kim back to the wellhouse, they find Mark dead. They go down to the Drowning Cave without the Owl.
When Ellie wakes up, she goes after them through the secret tunnel, bringing the owl. She makes the Owl snap the Shadow Crown off of Dodge’s head, making the shadows holding the cave up disappear.
Also Dodge uses the Owl to attack the kids on February 10.
Source: Locke & Key Wikia