Books. Books are powerful. They contain knowledge, and knowledge is power, believe it or not. Only some books don't just hold knowledge, but raw, unrefined power. An example, is a book named "Fate". The book allows a pony to write something in the book, letting destiny to be changed. You can write about somepony who you want to be your special somepony, and both of you kissing, and it will happen. That's just an example. It has a set of rules so ponies don't abuse it.

So, what happens when a sad, powerless pony finds this book? He turns into the opposite of what two traits I gave.

(Inspiration from the anime 'Death Note'.)