I'm searchin a story

Ok, I'm going to the point: I am searching a one-shot about Equestra under attack by an alien race. The One-shot focuses on how the Princess has a meeting with a member of a race that, the story leads the reader to believe, forces her to sign a surrender.

The final plot-twist is that this race is humans. But the princess does not sign any surrender, but an "alliance" since only this would save Equestria from extinction. I read it a while ago and have failed to find it. Aid! :D