Beelzebub (Manga)

The story revolves around a sadistic delinquent called Oga. One day when hes making a group of people kneel before him an old man drifts down the river behind him and gets washed up there. The old man splits in two and out comes a little baby that gets attached to Oga.
Then a demon called hildegarn chases him and tells him the baby is to be the new satan.