This is a MLP and Bioshock cross over story.

Big Mac is a Big Brother, and he is tasked with guarding his Little Sister, Apple Bloom, as she goes around to collect Spark. They wander through the ruined underwater city of Canterlot, and the whole place is a bitter shell of what it once was. Things are getting really dangerous, but still.... Big Mac has one job... to keep Apple Bloom safe.

Celestia has a dangerous creature held in the deepest of Canterlot's high-security dungeons. It can create ponies (with full backgrounds and histories) with it's strange and often-times prophetic powers. The Princess does her best to keep it under wraps, but its only a matter of time before it's "creativity" will get out of control.