The Alicorn Amulet - all powerful, and power corrupts absolutely. A great addition to any thief's collection.
Too bad Lock Pick, an earth pony thief, stole a copy of it that lacks wings. Having unicorn powers is amazing and all . . . but is it talking to him? It's sure made a mess of corrupting him - or she, as the amulet insists for him to refer to her as. And she's rather charming, in her own fumbling, naive, yet relentlessly-attempting-to-be-evil way.
Even worse, he's made the enemy of the entire Royal Guard because, while rather mediocre magic the amulet provides, it's still dark magic. And dark magic is kind of extremely-forbidden.
The entry of Imperaxum to the Straight Shipping Group's Romance Amidst Adversity Contest.
I'm making this up as I go.