Where is this story?!

Right, ages ago, I remember finding a story about an alternate timeline/crossover thing, where, at the end of Gravity Falls, Dipper essentially became Bill Cipher. Many years later, having had to watch Mabel and all his friends die, and very bored, he opens a portal to Equestria. Hijinks ensue.

However! Now, no matter what I search, I can't find it anywhere! What do I do?