If that's how magic works, would it be possible for Karnage to learn fire magic? Cause even though he's a tree, he has done some work with various resins and and oils that he can put onto things to light them on fire, but not do too much damage to the object itself. He discovered this when he arrived in a world where torches were not magically powered, but used some sort of residue that was flammable, but burned slowly and could even keep a flame in the rain. Despite that description it wasn't magical, just a chemical reaction to certain types of sap. Which he's going to look for or find a similar substitute, because nothing's more intimidating than a giant burning effigy coming to turn you into bone meal.