SO, let's place our bets! Will little miss M A) return to being just Maria, B) continue to waffle back and forth between Maria and Mao after some sort of balance is struck, C) be somehow separated into different bodies so both girls get to live, or D) the two will fuse into one person after the mental-emotional baggage that made Maria sequester everything that made her maou into a tiny corner of her mind and eventually gave it a life of its own.
My bet's on D, because thus far we've largely been dealing with a lot of internal conflict that through quirks of the setting end up manifesting externally. Arnest's inability to rely on others, Cu's loneliness, Maria's unease with being the daughter of the Maou (well, we don't know the specifics but Mao herself says she thinks she was born from Maria's unease roughly five years ago).
either way, Blade's in a tough spot here. He's at the weakest he's ever been and he just got told, "I want to kill you, let's become lovers".