The spell Justice Light used to hide his presence so the world would believe he had left with Juice Box.
Only two beings were left on The Ninth Floor at the end: Emberus, and Justice Light, who had used a spell to hide his presence.
That concealment was the whole trick. Everyone — including Juice Box, who he told he would follow into The Nothing — believed he was gone, which let him remain on this side of the veil long enough to snap the twine on his last trap.
The concealment that lets Justice Light stay behind unnoticed and spring the trap.