The ending was such a massive let down especially after all the build up. For anyone who want to know, the biggest plot point of the story is the identity of the demon king in the old timeline. It established that no one know who the demon king was, not even the demons. The guy just showed up one day, defeat the previous demon king and claim the throne. The strangest thing is that the demon king was hornless (demons' horns are a status symbol for them so a hornless become their king is strange). IDK if it is the same in the novel or not but in the manga, the backstory of the DK literally got reveal after the final battle and it basically Cell in DBZ, down to wanting to archives his perfect form by time travel to the past and merge with his something that only exist in the past. It was a massive letdown because the story built up the DK as someone who the demons race fanatically follow like a cult and he turned out to be just a strong guy.

Even the plot of the MC is kinda weak since he is so vanilla. Really the saving grace of the manga is the Seran's plots, how he is the bastard son of the king, how he has his own harem and romantic relationship with the current demon king.