Metamorphosis is the name of a stage illusion invented by John Nevil Maskelyne, but most often associated with famous escape artist Harry Houdini and performed to some renown (for speed) by The Pendragons, among others. It is also known amongst magicians as the Substitution Trunk (often abbreviated to Sub Trunk).
In the illusion, an assistant (Houdini employed his wife Bess) is locked inside a large box or trunk, often after being restrained with handcuffs, ropes, bags, etc. The magician stands on the trunk and holds a curtain up to momentarily conceal his entire body. When the curtain is lowered, it is now revealed to be the assistant standing atop the box, the magician and assistant having changed places instantaneously. When the box is opened, it is shown to contain the magician, restrained as the assistant had been. In some acts, the magician is locked in the box first, then appears standing atop the box.
Variations include the Aquarian Illusion performed by Mark Wilson, wherein a tank filled with water is used and a beautiful swimsuit-clad woman assistant is locked inside underwater, and Penn and Teller performing the illusion in an underwater theater with the female assistant handcuffed & locked in a steel cage on the bottom. These variations add an element of peril. If something were to go wrong, the assistant (or magician) could possibly drown.
Pictured to the right is "Suspended Animation". Called the modern day metamorphosis, the trunk is suspended on a platform above the stage creating an aerial exchange. This evolution of the illusion was invented by Australian Illusionist, John Taylor.
Illusionist Criss Angel assisted by Klayton and his other assistants has also performed the Metamorphosis trick as part of his "Amystika" show. In Angel's version, he was handcuffed, and put into the box with assistant standing on top of it. After the flashes and torches lit the stage, they swapped places.
A method for Metamorphosis was exposed by the Masked Magician, Val Valentino, as part of Fox TV series Breaking the Magicians' Code: Magic's Biggest Secrets Finally Revealed. He was assisted by Michelle Berube. However, the method exposed (specifically the manner in which the trunk is opened) is not generally in use today.