Mirror Shield

The Mirror Shield is one of the Three Sacred Treasures, kept in a Sacred Casket and kept guarded in a Fortress. Pit must recover this artifact and, combined with the Wings of Pegasus and Arrow of Light, take out Medusa in the Palace in the Sky. It appears as his down special in Super Smash Bros. Brawl.

Kid Icarus
The shield itself possesses a mirrored cover and protects Pit like a tower shield. Without this, Pit would be turned to stone by Medusa by merely looking at her, as the shield protects against her petrifying glance.

Kid Icarus: Uprising
The Mirror Shield, along with the other two Sacred Treasures, must be retrieved from a pirate ship in Chapter 8 before battling Medusa. After defeating the Galactic Fiend Kraken, Pit receives the Three Sacred Treasures in a handy case and uses them against his battle with Medusa later on. The Three Sacred Treasures are later broken apart by Hades in Chapter 23. The shield can block attacks that come at an angle facing the shield, similarly to how the Back Shield blocks attacks directed at Pit's back.

Super Smash Bros. Brawl
Pit's Down+B move that reflects any projectile, as well as protects Pit against any move, the Mirror Shield can fully protect him from damage when used properly. Pit's shield will, when used, be pulled from behind him; Pit will crouch down, and the shield will cover his front. He is still vulnerable from behind, however. Pit's shield can also be tilted to protect against attacks from above.

Trivia

 * In Greek mythology, Perseus was equipped with a bronze shield polished to a mirror-like sheen when he killed Medusa. It was given back to Athena, along with Medusa's severed head. Athena fastened the head to the shield, making the Aegis. This could have served as an inspiration for the Mirror Shield.