Okullos

One if the Four Grand Dukes.

He came back with Randalph.

He assimilated with the souls of the demons under him Such acts were forbidden in the Demon World.

He is also a demon ‘predator.’ A predator of his own kin...! He broke another taboo.

He summoned Nothingness by sacrificing Hundreds of thunderous human heads.

He was Killed by Randalph in Chapter 128.