Red Mirror is a FIST supplement that helps GMs to create high-powered enemy agents.
The PDF is 2 pages, with a clean, readable, and well-organized layout.
The rules in Red Mirror are simple and easy to follow, and produce direct foils to the PCs. Red Mirror foes have their own stats, apply penalties to PCs who interact with them, and have similar Traits---though used in different ways.
They also drop unique Traits when defeated---which is difficult, because their high HP and damage and the penalties they apply make them beastly hard to face. The Traits they drop are extremely strong, however, and make for a good reward when the PCs eventually managed to drop one.
Overall, this is a cool, impactful add-on for any game of FIST. It's not difficult to use, and it adds some very memorable fights to any other FIST content you're running. Pick it up if you have the chance.