The alicorn amulet

The Alicorn Amulet.

The Alicorn Amulet is a nigh-indestructible relic of dark power. It greatly enhances a unicorn pony's (and assumedly an alicorn's) magical capacity.

Breakbeat discovered it in the depths of The Everfreeze Forest, near the former location of Zecora's hut. She boosts her magical abilities with the amulet several times in order to help her friends, but the amulet quickly erodes her sanity, making her sadistic and vengeful.