Mirror of Truth
The Mirror of Truth is a magical relic which can create perfect copies of the body and soul of all who walk through it. Medusa and Pandora planned to use it to make copies of all the monsters of the Underworld Army and also a copy of Pit to serve them. Luckily the mirror was destroyed by Pit, but not before it created Dark Pit out of Pit's dark side when he broke it.
The Mirror of Truth is only witnessed in Chapter 5: Pandora's Labyrinth of Deceit. After the first part of Pit's battle against the goddess, Pandora warps towards the Mirror asking Pit to come and take. Instead, he destroys the glass with a karate kick, with Dark Pit being created in the process. Even though it seems that the doppelgänger is on the good side at first since he helps Pit defeat Pandora, he soon kicks him back. He then steals Pandora's powers and flies off. In the next chapter also known as Dark Pit, the Mirror is mentioned in a conversation between Palutena and Medusa regarding why the dark angel doesn't serve any side.
Said to reveal the truth, this mirror is also thought to be used by Pandora to transform souls into Underworld monsters. Pit destroys it with a kick, accidentally bringing about the creation of Dark Pit in the process.
Comparison showing earlier footage with the Mirror still intact as seen in the background (left), and the final product (right).