Lich.
They were beings, particularly those who pursue the path of mana, who had given their soul to a demon in exchange for a large mana capacity and long lifespan.
'So he's a lich, huh?' Regis thought silently as he saw the cracked marble-like thing embedded in the middle of Harold's heart.
'The logs that I found in that cave must've belonged to him- on second thought, I think that even that skeleton belongs to him too.'
'Still, a lich capable of casting master rank magic spells… if we met him when he still had his old body…'
Regis couldn't help but shiver at the mere thought of facing a Four Star Realm Master Magician at full power.
'Fortunately, he still hasn't fully assimilated with Harold's body. Coupled with the fact that his bond with his father seemed to be very strong too…'
At the thought of this, he glanced at the two corpses near him and sighed.