The lore behind it is that this thing called Black Fire (sometimes

Refered to as Shadow Fire or Death's Flame) is 30x worse than normal or even fire conjered by mana. Even the least flammable thing will go up in Death's Flame in mere seconds.

If you catch fire you're dead, it'll burn you alive in seconds, and it'll keep most of your corpse together and use as a zombie to spread the flames.

This had devastated Simon's world, and when he defeated the cause of it. A part of it lived in Simon, feeding off of his mana. It soon became a mutual relationship, at Simon's expense of his mana of course.

Some souls that still lurked pester Simon about using it (cuz now he can cast Shadow Fire) and since Simon is a Celestrial (weaker angel that guides lost souls into the afterlife - Dragon Quest IX) he can see and hear those cries. And it pains him that he can't release them from their pain because to do so he has to get rid of his Death's Flame, which has stuck to him like a scar.