The Eminence in Shadow, Vol. 12 (manga)

After casually defeating the big demon thing that showed up at Rose's wedding, Cid made a badass exit via the collapsing portal it came out of, but...is this Japan? How'd he and Beta wind up back on Earth? And what the heck happened while he was away?! Judging by the ruined city, piles of corpses, wandering monsters, and mana lingering in the air, it must have been a pretty wild party!