General: alicorn amulet
The Alicorn Amulet is a magical object acquired by Trixie in episode 5 of season 3: "Magic Duel." It is originally owned by an unnamed shopkeeper, but Trixie purchases it for a large bag of coins.
The Alicorn Amulet is a necklace, somewhat resembling the Elements of Harmony in basic design. It bears a diamond-shaped central piece ornamented with a ruby, two metal wings, and a metal unicorn head. When worn, the Amulet changes the wearer's magical glow to red and causes the wearer's eyes to glow red as well.
The Amulet is stated to enhance the wearer's magical power, and Trixie uses it to best Twilight Sparkle. She even performs aging spells with it, which Twilight says only the highest-level unicorns are capable of, and seals all of Ponyville under a magical dome of force. However, the Amulet also corrupts the wearer, and is implied to make them both evil and irrational. Under its influence, Trixie becomes paranoid and extremely hostile, but once it is removed she apologizes for her previous actions and seeks to make amends.
The Alicorn Amulet is currently in the possession of Zecora.