The devil's mirror

An evil troll (called the "devil") has made a magic mirror that distorts the appearance of everything it reflects. It fails to reflect the good and beautiful aspects of people and things, while magnifying their bad and ugly aspects. The devil, who teaches a "devil school," took the mirror and his pupils throughout the world, delighting in using it to distort everyone and everything; the mirror made the loveliest landscapes look like "boiled spinach." They tried to carry the mirror into Heaven with the idea of making fools of the angels and God, but the higher they lifted it, the more the mirror grinned and shook with delight, and it slipped from their grasp and fell back to earth, shattering into millions of pieces. These splinters — some no larger than a grain of sand — were blown around and got into people's hearts and eyes, freezing their hearts like blocks of ice and making their eyes like the troll-mirror itself, seeing only the bad and ugly in people and things.
- from The Snow Queen

I've been watching an episode of Once Upon a Time in which the evil Snow Queen has cast the Spell of Shattered Sight on the people of Storybrooke, causing tiny pieces of a broken enchanted mirror to get into their eyes and make them see only the worst in everyone else, even those they love :( ...