"A key given to you by Lilly.
It opens the door to the garden."
The Garden Key is an item found in Prisoners of the Elder One. It is given to Erik by Lilly after giving her any flower (excluding plastic). Since the bedroom can be unlocked with the Room Key, this item is not mandatory for any of the games' content or endings. However, one must give Lilly 3 flowers to get the C-4.