I was curious about the Heat Metal spell. I have a Cleric in my party that has been using it to add to his damage by casting it on his weapon. Does the attack he makes deal added damage instantly or does he use a bonus action to add it? Or does it work that way? If you cast heat metal on a weapon you're holding, you take the damage.
— Jeremy Crawford (@JeremyECrawford) 7 maggio 2018
It’s interesting that the spell doesn’t technically distinguish between wholly metal objects and those, like axes, that are only partially made of metal.
Does “manufactured metal object” mean an object entirely comrpised of metal? Does it mean an object substantially, but not wholly, made of metal? We don’t know – the rules simply don’t say, and WotC’s beloved usage of “natural language” makes this spells ruling incredibly vague and infinitely debateable.
The spell does specificy that a creature takes damage if it contact with the object. That tells us that if it DOES work on an axe comprised of a steel head on a long wooden handle, it deals damage to people even if they are touching the wood and not the metal itself. Which is… odd.
They could have clarified the problem entirely by including a single line or phrase of text in the rules. Either narrow down what kinds of objects are eligible to be targets to only include wholly metal objects, or restrict damage to only creatures in direct contact with the metallic portions of an object.
But, for some unfathomable reason, they didn’t.
Coming back to this again, it occurs to me that there’s nothing actually stopping someone from using Heat Metal to deal extra damage to enemies while attacking.
I mean, sure… you’ll take damage. But if that’s a cost you’re willing to pay, or one that you can circumvent, I see nothing in the way the spell works that would prevent being able to also damage the enemy when striking them with the affected weapon.
Attack Action to swing at an enemy, and when you hit you use your Bonus Action to deal damage to “any creature in physical contact with the object”.
If you have Resistance to fire damage, then you’re dealing twice as much extra damage to the enemy than to yourself, which it might be worthwhile. And of course, if you have full Immunity to fire damage, then there’s no real downside. (Aside from opportunity for obtaining said Immunity in the first place.)
That said, in most cases by the time you have access to a source of Immunity, dealing 2d8 damage on a bonus action will be of somewhat reduced value. You would probably get the most value out of it at lower levels by settling for mere Resistance to fire instead, which is much more easily attaned.
The spell doesn’t deal damage to anyone besides the one holding the weapon, unless you can somehow convince an enemy to let you keep it in contact with them after you completely finish making your attack.
When you take attack action, you can attack, move, attack again (if you have multiattack). Dual wielding, you can use the bonus attack to attack with second weapon but not the first, so it doesn’t feel to me like the action and bonus action are always strictly sequential (if at time of bonus attack you had already finished the whole attack action, why couldn’t you do it with the same weapon?). It doesn’t feel impossible to me to use your bonus action at the moment the attack concludes in a hit.
What is obvious though, is that you can do the bonus action damage only once (because you have one bonus action), so multiattack would not apply it to every attack (thus it would function more like sneak attack, you’d apply it on one hit).
Given that you’d need to somehow deal with the damage to yourself – and possibly dropping the weapon or suffering penalties, and in many cases you could simply cast it on metal piece worn/wielded by enemy anyway, I think being able to use your bonus action to deliver the damage through one of your attacks would not be unreasonable.