A strange effect will activate when Ch.X is cleared!
Even though the Mystery Mask is the last Treasure set in the Cats of the Cosmos Treasure menu, it is not the last the player can complete - that honor goes to the Neutrino. It reduces the 1100% strength magnifications applied to the final boss of its respective COTC chapters by up to 1000%. Enemy stats shown on this wiki are with completed Mystery Masks. Its effect is to greatly weaken the final boss of the chapter (to 100%), which makes it almost essential as without it, the boss will be buffed to 1,100%, making it impossible to defeat.
When a player has activated a Cats of the Cosmos Chapter's Mystery Mask, the boss of The Big Bang will be weakened by up to 1000% (additively). For example, Chapter 1's final boss has 1,399,999 health and 53,997 attack power, but the default stat magnification applied to his base stats without the Mystery Mask is +1,000% upon his initial 100% strength magnification, making him 1,100%, with a health and attack power of 15,399,989 and 593,967 respectively.
The Mystery Mask from any one chapter DOESN'T affect the final bosses in the other two chapters.