The party is facing off against a massive stone giant. Aidan, the group’s fighter, fires from his longbow while the giant is still at range. The attack hits, thwumping the giant in the leg for a few points of early-combat owwie. Next to go is the bard, Avonia who rolls to seduce the giant casts Heat Metal, targeting Aidan’s arrow, dealing 2d8 fire damage immediately, and hitting the giant with a debilitating debuff. The plan is to continue triggering Heat Metal as a bonus action on subsequent rounds to re-apply the damage and debuffs until the giant is able to pull the arrow from its leg.
Does this plan work?
Let’s start with the text of this second level Transmutation spell:
Choose a manufactured metal object, such as a metal weapon or a suit of heavy or medium metal armor, that you can see within range. You cause the object to glow red-hot. Any creature in physical contact with the object takes 2d8 fire damage when you cast the spell. Until the spell ends, you can use a bonus action on each of your subsequent turns to cause this damage again.
If a creature is holding or wearing the object and takes the damage from it, the creature must succeed on a Constitution saving throw or drop the object if it can. If it doesn’t drop the object, it has disadvantage on attack rolls and ability checks until the start of your next turn.
Avonia’s attempt is going to fail because the spell can only target metal objects the character can see. The arrowhead is stuck deep in the giant’s flesh, and the shaft of the arrow is made out of wood.
What if Aidan had used an arrow with a metal shaft instead?
The shot would have some massive penalty applied, something worse than ordinary disadvantage. Arrow shafts need to be flexible in order to be stable in flight (watch any slow motion video of an arrow being shot). A rigid metal arrow is not a practical weapon.
What if Aidan knocks an arrow, and then instead of firing, readies the attack action which will trigger when Avonia casts Heat Metal on the metal arrowhead?
Rule as Written, this would work, though the damage and penalties won’t apply until Avonia’s next bonus action.
Should it work though?
Yeah, this is fine.
“The stone giant feels a sting in its leg, and rips the arrow out before continuing its charge at the party.”
Depending just how strongly you want to discourage this idea, the giant could pull the arrow out as just a simple free object interaction, no more challenging than pulling an arrow from a quiver. Or, it could be given a modest strength or constitution save on the attempt (but this is before Avonia has a chance to start applying disadvantage on ability rolls; the heat won’t flare up again until her next bonus action).
Or, it could take a full action to remove the arrow, meaning the enemy loses its action at the cost of a player’s action and spell slot. That could be a balanced trade.
Full Metal Bolt
What if Aidan instead used a crossbow with a solid metal bolt? Unlike an arrow for a bow, bolts are thicker and heavier and don’t need to wobble through the air.
He’d likely have to get the bolt custom made, and this could impose a reasonable cost or use limit on this combo. After several rounds of being heated red-hot and thrashed around through combat, the bolt may be warped beyond use. But, a dagger is only 2gp, and 20 arrows 1gp, so realistically a small sack of gold is going to easily buy a big enough supply to overcome this limitation.
What could instead work is to have the metal bolts reduce the range of the weapon and remove the attacker’s proficiency bonus while firing them (because the character is not proficient with this unusual, heavy ammunition).
Perhaps that’s enough of a tweak to balance it out, but really the question is going to be how difficult it is for the enemy to remove the bolt, assuming they can reach it.
DC to Remove
Use your own judgement, but I think this gets in the ballpark:
A creature hit by this blazing full metal crossbow bolt may use its free object interaction to attempt to remove the bolt. This is a constitution save made against a DC equal to 10 + the damage die’s roll (without modifiers). Alternatively, it may use its action to attempt to remove it, in which case the DC is 5 + the damage die’s roll (without modifiers).
The DC represents the idea that the thwumpier the hit, the harder it will be to remove the bolt, and building variance into the picture makes this less of a reliable go-to strategy against a single large enemy, but occasionally the incredibly effective tactic the players are hoping for.
Even with a +5 to constitution save and against a low DC, the stone giant Aidan and Avonia are attacking will probably want to spend a full action to make sure that bolt gets removed. (In this scenario, Aidan sinks the bolt, and then Avonia casts Heat Metal, meaning the giant is already suffering from disadvantage.) That makes this still a very powerful combo, essentially giving the party a free round of damage against the enemy. And keep in mind, this is how strong Heat Metal is against an enemy without any metal gear.