KMST 1.2.472 - Wind Archer (II), BGMs

The Black Mage really liked Demon Slayer. I still think Arkarium did it on his own and just claimed he was under orders from the Black Mage.

yes, it was indeed arkarium's own doing. He hated Demon Slayer because the Black Mage preferred him over Arkarium.

It might have actually been the Black Mage, seeing his new favorite and possible strongest commander is Demon Slayer very own brother.

Damian kills his mother, joins the black mage, and goes to capture the World Tree while the battle of the heroes and the black mage happens. That seems more possible than Arkairum doing it on his own, he is a coward.