I imagine that the breath of a dragon is a power that is resisted and the bite from a dragon is not. This brings some problems though... imagine the fire is as real as the air a dragon breathes - would that still be resisted. If a dragon spat on you it would not be resisted, right? But if it spits fire it is? Or perhaps dragons fire is not resisted at all?
I'm not sure how canon this is, but I can see two ways to model the dragon's breath. One, you design it as a power; it has a Might cost, it requires Penetration, etc. Two, you call it another natural form of attack, give it Init, Atk, Def, and Dam stats and away you go. No Penetration needed.
I'd bear in mind the nature of fire though. You might want to look at how you work out the Damage Total though. Fire is usually considered "non-combat" damage, which means you roll Stress (I think) + the damage modifier and the victim rolls Stress (I think) and adds their Soak Score to get their total. Not sure how that works (or if you need it to) if you simply treat the flames as just another weapon. It's just a vehicle for applying damage, anyway.
It can ignore resistance if you want, but then it needs a finesse roll, or arguably some other form of ability roll. No auto-targeting. But magic powers can totally duplicate ritual effects despite what RoP: Magic tells you. The general assumption is that powers auto-hit and are resisted.
Thank you. I will probably rule that dragons Fiery Breath is normal (given that the mentioned can breath fire) not resisted and that a targeting roll is required. A dragon can probably also use its breath a limited number per day. This is how the dragons Fiery Breath were in the early editions ArM1-2, but in the later editions it is not clear. In ArM5 Fiery Breath is a power costing Might but you can still dodge it. In Calebais is is a power but no roll to hit is required. An error?
I'd do it the way Mark suggested, although I'd just incorporate the fire damage in the Damage calculation of the attack. If it's a natural attack, it doesn't need to penetrate, nor does it need any sort of potentially awkward targeting roll. It's a flaming missile, essentially. If the dragon can only do this fire attack a certain number of times, just decide how many times a day it can use it and keep track of it. I wouldn't make a special rule for that.