From publisher blurb:
Outcasts from their homeland, they fled into the ocean and discovered the ruins of an ancient people -- and awoke an ancient evil. Now they return, to conquer their old home and bring with them the power of their dark lord. They are the Muan, masters of the Undersea Shogunate of Mu. They control 7/10 of the world, and now seek dominion over the rest in the name of Dagon, Dark Father of the Sea.
"The Undersea Shogunate of Mu" is a Fate Core supplement for the Mecha vs Kaiju setting, but can be adapted to any other system. This supplement includes:
A complete history of the Shogunate, told from their point of view
Rules for adventuring in 1970s Japan, including the burgeoning rivalry between the Anti-Kaiju Force and the Mecha Assault Force
"The Promise - The Power - The Price" or "Selling your character out to dark elder gods"
Stats on the Emperor/High Priest of Dagon, the Shogun of all the Seas, their army and priesthood, and the living war machines which make their conquest possible
Allies and enemies of the Shogunate
A complete campaign culminating in the Shogunate's invasion of Japan