It looks like fire, it crackles like fire, and it even burns like fire. However, it isn't really fire. It looks like fire because that's what the Master of Illusion wants you to see. Despite crackling, it consumes no air, burns no fuel and creates no smoke. If it generates heat or burns victims, it's because it's magical or because Your Mind Makes It Real. Even if it can incinerate a chair, it rarely spreads to the rest of the room. It probably deals less damage than straight-up pyrokinesis.