Arcane Archer's Piercing Arrow on Larger Enemies

Question

One of the Arcane Shot options for the Arcane Archer fighter (from Xanathar's Guide to Everything, p. 28-29) is Piercing Arrow. The description of Piercing Arrow is as follows:

Piercing Arrow

You use transmutation magic to give your arrow an ethereal quality. When you use this option, you don’t make an attack roll for the attack. Instead, the arrow fires forward in a line, which is 1 foot wide and 30 feet long, before disappearing. The arrow passes harmlessly through objects, ignoring cover. 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. On a successful save, a target takes half as much damage.

My question specifically is how is this handled for enemies that are large than your normal 5'x5' square? A Tarrasque is said to be 70' long, and if the arrow goes 30', then it would pass through 6 5'x5' squares before it's done. RAW it seems to say it would just hit it a single time.

So versus a large monster, would this basically be a single shot against a large creature (bow damage + 1d6 piercing) and is more tailored to a line of smaller enemies?

Also, would it be too much of an imbalance to have a creature do multiple saves and damage if it passes through multiple squares' worth of its space? (I'm thinking of something like the Piercing Ammo or Dragon Piercer from Monster Hunter World.)

Answer 1

RAW, only hits once

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.

If the creature is hit, it takes the damage as stated. Arguably, a Fireball would do more damage to a Huge Treant than a Small Vegepygmy since there's more surface area to burn, but as a general rule, each attack roll only hurts a creature once. So, you're right that it's more suited to a line of weaker enemies.

As to balance, multiple hits is likely overpowered

Huge and Gargantuan creatures tend to have poor Dexterity saving throws. From point blank (up to 6 separate "hits" on the same creature in the line). With a small chance of making the saving throw, it's likely that the creature will be hit with 3x-4x the typical damage, especially considering that the creature's AC is being ignored. This will unbalance encounters, essentially making a party far stronger against Huge or Gargantuan creatures while staying at a similar power level against groups of smaller creatures.

Answer 2

In D&D 5th edition, it is lead designer Jeremy Crawford's stated opinion that a rule says what it says and no more. Therefore, when an ability says you strike every creature a line once, you strike each once no matter how big they are.

This means that, unless the DM deploys a variant rule, a piercing arrow will hit the Tarrasque once only.

Increasing the damage for larger creatures would make the ability substantially more powerful. A gargantuan creature would take 4d6 bonus damage, equivalent to a Grasping Arrow or Shadow Arrow by an 18th level character, at which point the Piercing Arrow would itself deal 2d6 base damage rather than 1d6. In short, it's considerably too powerful, even if it does make more sense or seem more realistic.

If you did employ such an ability, it would be more straightforward for the creature to roll a single saving throw, not one for each five-foot chunk of its body that the arrow passes through.