Changeling ability to become a non-corporeal wraith, letting Juice Box pass into a sealed room and possess Louis.
To reach Louis in his sealed room inside The Reaver Castle, Juice Box turned herself into a hunter wraith, a floor 10 and 11 monster. Tipid warned it would not be a clean getaway because of what happens when that particular wraith touches people, though he judged that a problem for Juice Box rather than for Louis.
The plan relied on wraith possession: people who are wraith-possessed can be moved like puppets even if they are dead, and the wraiths can and do use items from the crawler's inventory, which is what allowed Louis's No Disassemble EMP Bombs to be dropped and his teleport to be triggered. Carl calls it a common and terrifying mob from the higher floors. Juice Box feared possessing Louis might kill him, but it worked, and she was able to sense the presence of Epitome Tagg inside his replacement organs through the connection.
The mechanic that makes the entire Louis rescue possible.