dnd 5e – What counts as damage by an ammunition when using Arcane Shot in regards to poison?

The effects of the poison happen after the effects of an Arcane Shot

Arcane Shot options are applied “when the arrow hits a creature” meanwhile the poison happens “when a creature takes damage”. Whether getting hit and taking damage are simultaneous events is unclear, and thus, whether these are simultaneous or not is unclear. Furthermore, the exact timing of “when” in these features is similarly unclear.

However, we can see that most Arcane Shot options cause the attack to deal extra damage, meaning that damage is still part of the same event, the same damage roll. This can be seen from how critical hits interact with extra damage, in particular, the fact that Sneak Attack, which does “extra damage” is doubled on a critical hit, and thus the Arcane Shot options would be as well.

Therefore, I would conclude that we actually do know some of the order of events:

  1. You hit the creature
  2. You choose to use an Arcane Shot option
  3. The damage of the attack and Arcane Shot are rolled together
  4. The poison is applied (assuming the target takes damage)

Now to address some of the specific cases:

A Bursting Arrow only poisons the initial target

Bursting Arrow states (emphasis mine):

(…) The energy detonates after your attack. Immediately after the arrow hits the creature, the target and all other creatures within 10 feet of it take 2d6 force damage each. (…)

Meanwhile, the Poisoner feat states (emphasis mine):

(…) Once applied to a weapon or piece of ammunition, the poison retains its potency for 1 minute or until you hit with the weapon or ammunition. When a creature takes damage from the coated weapon or ammunition, that creature must succeed on a DC 14 Constitution saving throw or take 2d8 poison damage and become poisoned until the end of your next turn. (…)

The poison loses its potency the moment you hit something, which happens before the effects of Bursting Arrow. Therefore it does not apply its poisoning effects to the creatures damaged by the burst.

A Grasping Arrow only deals poison damage once

Grasping Arrow states (emphasis mine):

(…) When this arrow strikes its target, conjuration magic creates grasping, poisonous brambles, which wrap around the target. The creature hit by the arrow takes an extra 2d6 poison damage, its speed is reduced by 10 feet, and it takes 2d6 slashing damage the first time on each turn it moves 1 foot or more without teleporting. (…)

One reason Poisoner cannot apply to the movement-induced damage is because it is from conjured, magical, grasping, poisonous brambles which are not part of the ammunition itself. Another reason is because the poison’s potency is completely gone once the arrow hits the target, so the poison is gone by the time the target moves.

A Seeking Arrow poisons the target normally

To use the Seeking Arrow Arcane Shot option, you must declare that you are using it before you make an attack roll. The shot option states:

(…) When you use this option, you don’t make an attack roll for the attack. (…)

Then the arrow strikes the target, whether they succeed on the saving throw or not, which forces them to make the saving throw against the poison.

A Piercing Arrow… does something? Ask the GM?

Piercing Arrow states:

(…) Each creature in that line must make a Dexterity saving throw. On a failed save, a creature takes damage as if it were hit by the arrow, plus an extra 1d6 piercing damage. (…)

Meanwhile, the Poisoner feat states (emphasis mine):

(…) The poison retains its potency for 1 minute or until you hit with the weapon or ammunition. When a creature takes damage from the coated weapon or ammunition, that creature must succeed on a DC 14 Constitution saving throw

Notably, the feat assumes you are damaging only a single creature, so what exactly happens when multiple creatures are damaged at once is, at the very least, unclear.

Personally, I would not let an Arcane Shot option expand the number of targets you can effect with a poison when poisons are designed around being single-use effects. I would rule that the poison effects only the first creature in the line of a Piercing Arrow. A GM is free to rule otherwise, and I can’t see much in the rules that makes this any clearer.

A small note about the Seeking Arrow and Piercing Arrow Arcane Shot options: The poison only loses its potency when you hit something and it’s possible you haven’t actually hit anything in these cases. The exact meaning of the “as if it were hit by the arrow” phrase found in both options is not clear; does this remove the poison from the ammunition?

Personally, for me, it would remove the poison because you have used up the poison damage, and its effects should end just like any other time it does its damage. You should not be able to pick up the arrow and repeatedly apply the poison by doing this again.