JeffB
Legend
What's not established is whether and how this fictional position has any meaning at all on the game.
Sure it does- what do you do now?- try to get the flames out? How do you do that with the Hell Hound now holding onto that arm? Either way you now have a choice to make, your arm or burning flesh and face....and then your defy danger roll may cause other complications. Maybe the PC decides to worry about his arm more than his face. His next roll could save his arm and the flame goes out- Or save his arm and leave him permanently scarred which I'd probably give a perm debility to for CHA and temp one for being unable to see out of one eye. Or perhaps he worries about the flame, rolls poorly and now and the bites the PC's hand off or permanently disables it- either of which can completely alter the course of the adventure. If he completely fails, it could be both. and other than the initial 5HP, I wouldn't bother using the "deal damage" GM move anymore.
Your fiction and the spoken word triggers moves. what the player says, tags, current circumstances, and the die roll suggests the next course of action which is as mentioned elsewhere in this thread- snowballing dangers unless you roll extremely well.
It's a completely different headspace than D&D. And this scenario would not be possible under any modern version of D&D because it breaks so many rules. In OD&D I do stuff like this, however the Binary outcome of D&D still makes this an unlikely situation not to mention Hell Hounds only breathe or Bite in OD&D- they don't have a grab attack or an exhale function upon a bite. Sure you can break or create the rules to make the fiction better but then you are just making the game more like DW, and then the questions are-
Do you now you need to make rules for these Hell Hound abilities going forward?
How many aions can the Hell Hound perform per round?
What are the rules for maiming and burns?
How does Cure Light Wounds work on this?
If I give these things to a Hell ound how does this affect the XP value?
This has been the age old issue- eventually you end up adding so many rules that they become the straightjacket to the fiction and for no real good reason other than to have rules for every corner case. Dungeon World frees the DM to determine that balance, that severity of consequences, and a whole host of other issues in the moment without having to make rules exceptions to a strict set of rules.