The hero character gets some sort of apparel or accessory that seems to be great and useful, but it's really unstable, or sentient, evil (or at least mischievous in the way Muggles would consider evil).
In the case of a sentient, evil item, it often seeks to turn the hero to its own ends. Toward this goal, it begins manipulating the hero's mind. He becomes less sunny and more grim. He becomes [...] [tvtropes.org]