Based on the Season 5 Finale Changeling universe.

Equestria has fallen.

After striking down her own sister, Princess Celestia fell into a state of great depression, leaving her little ponies to fend for themselves. After a royal wedding ends in a hostile invasion, it's not long before all of Equestria falls to this new threat. Nearly everypony is taken prisoner, and forcefully fed off of to sustain the ever-growing changeling population. The few who retain their freedom under the changelings are ruled by the Queen's iron hoof, and are slaves in all but name. For those who have evaded the changelings, their reality is constant hiding and perpetual fear of discovery. None hold out hope of fighting Equestria's new masters.

Only one pony remains who can stop the changeling menace. And she doesn't think the changelings stand a chance.