Revange of a Hero

Ren sacrificed everything that needed to be sacrificed, even his happiness, just so he could kill the demon king. After dealing the final blow

Ren's companions felt him a fatal blow.

Angered by this unjust and unprovoked betrayal Ren vowed to get revenge on everyone who betrayed him if he got a chance.

Hearing his words, Satan made him an offer he couldn't refuse.

Unbeknownst to Ren, he was sent back into the past and there, he discovered many secrets that led to his fatal betrayal.