I remember starting this series a long time ago and then lost track of it while waiting for updates.

The plot goes - basically - the Demon king dies and his heir is sent to earth to be raised by humans. The heir doesn't know anything and suddenly he starts turning into a demon. Some loyal demons of his father help him out, but the "bad" demons start causing problems. I remember a certain arc where the entire school is killed and the demons chop up the MC childhood friend/love interest and separate her parts in different demons. The MC can save her if he can find all her parts before the time frame ends. I think he had a friend that runs away from him when he tells him about being a demon and he becomes a warrior of justice or something like that, I think. His other friends become protectors and get special powers or something as well.

That's all I really remember... Any ideas?

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Beelzebub?
like there was a school, good and bad demons, a friend who gets chopped up, the entire school gets turned to stone
but the Demon King is not dead, but this is the closest that i can think of

AHHH!!!! I've been looking for this one too. I remember this too, just can't think of the name...Didn't it come out pretty recently?

Goumaden Shutendouji
Its a shame it hasn't seen a translation release in 2 1/2 years

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Is this me in a nutshell?
Pretty much.

This is the one.... Damn, it's still not updated?! ARGH!

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On the plus side, Hokuto no Gun & HappyScans are (slowly) making their way through the complete volumes of Shutendouji, which is the main story from which Goumaden apparently branched from. Once they're finished, I'm sure they can be convinced to pick up Goumaden, if they can find the raws.