Mixi’s XFLAG announced the next special collaboration event for the Monster Strike users. From February 16, 2017 you can enjoy the Final Fantasy event in the Japanese version of the game.
This collaboration is the part of the 30th Anniversary FF year where you can get extra Dragon stones in the game and of course you can unlock Final Fantasy characters. The BGM also going to be different in the game’s main menu.
XFLAG already opened the official collaboration website of the event what you can find in here: https://www.monster-strike.com/promotion/ff/