Demon Reach has Bane Fire as a last ditch effort to destroy the prisoners if an escape happened. The fire is described as crimpson, and blue becoming white (if memory is accurate)... This to me sounds like Hellfire, and Soulfire combined... Like a matter, and antimatter explosion or something. With Harry having access to Soulfire, and possibly Hellfire.... I wonder if he could wield the power to kill Immortals... I mean if Bane fire does it to the prisoners why not a Bane Fire spell?
Perhaps this is why he is a weapon? Perhaps Starborns are destined to wield this kind of power. Mortals who can kill gods. I believe Merlin was called "The Conquerer" in Battle Ground, and Morgan was worried about Harry being a Destroyer with a capital D. Maybe that's what this is about?