Priscillia's Crystal

Priscilla's Crystal,which was created by Ethan, has the power to take the energy away away from another source and amplify it. Ethan made it so Priscilla could transport between kingdoms, but she was using it to drain the power of imprisoned monsters so she could transport her army to the human world and take over.

In the end, after Ellie and Chomp defeat Priscilla, there was only enough power for one person to leave. Ellie left to return to the human world while the Kid Kings stayed behind.