A meteor impacts the nation of Equestria, Planet Equis. Tranquility is quickly snuffed out when a poison begins to spread violently. The Princesses cannot combat this unknown threat, as no knowledge of it exists. Help comes from an unexpected source.
A machine built for exploring new worlds and preparing them for possible coloization. A land of magic and friendship that has not seen the blight of war in over a thousand years. This is what happens when the two come crashing together.
MLP/Warhammer 40K crossover, part 5. The pieces are placed. The battle lines have been drawn. The 38th Company and Equestria fight relentlessly against the alien menace (the other one, that is), but the odds are hardly in their favor.
An Evil (the capital E is important) long forgotten to the world stirs once more, summoning forth a new Overlord that the races and kingdoms of Equestria and beyond have never before experienced.