Grandline Shinsekai: Episode 21

Back in Black is the twenty-first episode of Grandline Shinsekai.

Plot
Believing Beelzebub to have been dealt with, Romeo and Silvia wonder the halls of the fake site of the summit. Romeo deduces that their current location is in fact a trap set by Macbeth. A voice inside his head confirms as much, the voice of Beelzebub. Shocked by the return of the demon, Romeo struggles to fight the demon inside his mind. After tormenting Romeo, Beelzebub releases a poison in his body that threatens to kill Romeo.

Romeo is saved by a second voice in his mind, the voice of a mysterious, and yet so familair man. This voice purges Beelzebub from Romeo's body, and takes over Romeo's mind and body, evidenced by the black marks that begin to cover his skin. Possessed, Romeo brutally batters Beelzebub before ripping his head off. Afterwards, he confronts Silvia and learns that she is evil, like the man who has possessed Romeo. He offers her a chance  to team up, leaving her the power to summon him, if she'd ever need it.

After their agreement, the mysterious entity returns Romeo's body to him. Romeo suffers from short term memory loss, and Silvia lies to him about the events that transpired, allowing them to move on.