The Apex Parasite nearly wiped the town of Truth or Consequences, New Mexico, off the map. With the brave efforts of Rainbow Six's operators, the outbreak was eliminated at the source, and the parasite destroyed.

That was a week ago, and the town is still under quarantine. However, when Rainbow Six traces the origin of the parasite-bearing capsules, they find Apex has reached farther than anyone could've guessed. The parasite has reached Equestria, crashing down in Griffonstone, Ponyville, and Our Town. The parasite quickly overwhelms all methods of resistance, leaving the ponies near extinction. Only Rainbow Six has the tactics, gear, and skills to take the parasite on a second time, to stand fast with the endangered Equestrians.

Operators, prepare to attack.
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Rainbow Six Siege crossover, set after the ending of the Outbreak mission. Will have spoilers if not completed. Will have swearing.

My co-writer is This amazing guy, or EvAn Ze Geek!