Being a prisoner of the No Zone meant Mephiles has a lot of freedom. And he means a lot of freedom. As long as he doesn't do what he did 10 Years ago with Iblis, he has pretty much free reign about the timelines. He answered everything he questioned, and yet he still wanted something.
The Faust inspired crystalline spirit decides he needs to create a Shadow. But, a terios of Shadow. And, out of everything in the multiverse, he chose Twilight Sparkle. However, she wouldn't just be the plain old unicorn. Mephiles would ensure she'd be the best he can make her.