Final Fantasy games on mobile tend to fall into one of two categories.They're either expensive-for-mobile ports of the console classics or free-to-play exercises in nostalgia, pulling characters together from throughout the series in the hopes that people will shell out money to chase their favorites. Dissidia Final Fantasy OO  [Free] belongs more to the latter category, but it's also decidedly less cynical than you might expect and could even rate as a real treat for those who simply can't get enough of the franchise.
Though it shares the first word of its title with the console fighting game Dissidia Final Fantasy NT, OO (which stands for Opera Omnia, in case you need that info to win a trivia contest sometime) goes its own way in terms of gameplay experience as a traditional RPG and tells its own story to boot. The backstory here involves warring deities, who have built their own world and forces warriors from many other realms to fight nonstop. Finally worn out, they discover a safe haven of sorts only to find that mysterious portals called Torsions are letting in monsters that threaten to spill back into the connected worlds and destroy everything. The less nasty of the two gods, Materia, charges one Mog with the task of rounding up heroes of light to turn back the monsters, seal up the Torsions and set things right.
It's actually a pretty good setup to introduce characters from various Final Fantasy games and get them to team together for the common good, to ...