After consulting a few trusted friends, I've confirmed that this mod can't be illegal because it doesn't work for people who own only FoTS.
What does it do? It ports over the units from RoTS and S2 into FoTS' campaign, making for a very exciting experience for traditional players.
I didn't port over naval units for obvious reasons.
Spoiler Alert, click show to read:
Simply unzip, place into directory as it is a movie .pack file.
YOU MUST HAVE SHOGUN 2, ALL UNIT DLC, ROTS, AND FOTS TO BE ABLE TO USE THE MOD TO FULL EXTENT.