Kail has been shown to use demon blasts during the tournament and that suspicious black winged demon lord motif when he killed the king after saving the princess during the hydra incident.
It's almost guaranteed that Kail is either a Demon (unlikely). . . .  or he brought over the Black Winged Demonlord or some Demon's soul from the last time period. 
Remember, Kail has two souls, but who said it had to be two copies of Kail's soul? Why can't one be from the Black Winged Demonlord. Or some other Demon's