Deflect Missiles against unusual projectiles?

Question

There are a number of monsters that make ranged weapon attacks with unusual qualities. For example the Spined Devil has a ranged attack that does 1d4 piercing and 1d6 fire damage, the Drider's longbow does an extra 1d8 poison damage, and the Sprite's shortbow has no damage die but can poison an enemy or knock them unconscious.

When using the monk's Deflect Missiles feature to make a ranged attack using a caught projectile are additional damage types and effects included with the attack?

Deflect Missiles

You can use your reaction to deflect or catch the missile when you are hit by a ranged weapon attack. When you do so, the damage you take from the attack is reduced by 1d10 + your Dexterity modifier + your monk level.
If you reduce the damage to 0, you can catch the missile if it is small enough for you to hold in one hand and you have at least one hand free. If you catch a missile in this way, you can spend 1 ki point to make a ranged attack with the weapon or piece of ammunition you just caught, as part of the same reaction. You make this attack with proficiency, regardless of your weapon proficiencies, and the missile counts as a monk weapon for the attack, which has a normal range of 20 feet and a long range of 60 feet.

This question on catching poisoned arrows is related but does not answer my question. The linked question asks about which damage needs to be reduced in order to catch the arrow. My question is about the damage that the arrow would do if the monk spends 1 ki point to make an attack with it.

For a specific example: A Drider hits a monk with an arrow and deals 4 piercing damage (1d8+3) and 1 poison damage (1d8). The monk uses Deflect Missiles and reduces the damage by 35 (1d10 + 5 + 20) lowering the damage to 0. The monk then spends 1 ki point to make a ranged weapon attack with the caught arrow as per the Deflect Missiles. The monk hits with the attack. How do I calculate damage?

Answer 1

Damage as an improvised weapon unless it is a thrown weapon

A spine is not a weapon and neither is an arrow. As such they will be treated as improvised weapons.

At the DM's option, a character proficient with a weapon can use a similar object as if it were that weapon and use his or her proficiency bonus. An object that bears no resemblance to a weapon deals 1d4 damage (the DM assigns a damage type appropriate to the object).

as such (like Aldean Zed stated) it will largely be up to the DM to determine the damage unless the it is an identifiable thrown weapon (such as a dagger) or the monk uses their martial arts die instead.

I would suggest that a DM could reasonably consider most projectiles deflected by Deflect Missiles to be sufficiently similar to the attack listed in the enemies stat block that they could be treated as the same.
The Spined Devil's spines could therefore deal 1d4+DEX piercing + 1d6 fire. The Drider's longbow attack could be deflected for 1d8+DEX piercing + 1d8 poison. The sprite's arrow could be deflected for the martial-arts-die+DEX and the chance to poison.

For any of these, however, the DM could rule the projectile as launched by the monk is not sufficiently similar and would do different damage. The Spined Devil's spine might only be alight a short while and so would not do additional fire damage when deflected. The Drider's poison might be wiped clean by the monk's catching and so not apply. The sprite's arrow might be able to poison due to spritely magic in the sprites bow that is lost when the Monk touches it. If the monk doesn't choose to use their martial arts die the DM has definitive say (more so than usual).

^{Note: Thank you to HellSaint with this question that helped make the issue much clearer for me.}

Answer 2

Let's apply the rules literally to your examples and see what happens.

Tail spine attack

Suppose a monk uses Deflect Missiles to reduce the damage of a spined devil's tail spine attack to zero.

Tail Spine. Ranged Weapon Attack: +4 to hit, range 20/80 ft., one target. Hit: 4 (1d4 + 2) piercing damage plus 3 (1d6) fire damage.

This is a ranged weapon attack, so it can be deflected. The devil is small, so its spine is certainly small enough to hold in one hand, which means the monk catches it. They then spend one ki point to throw the spine at an enemy. As described in the monk ability, for this attack, the spine is considered a monk weapon, conferring all the benefits that go along with that: the monk makes uses their dexterity modifier for the attack and damage modifiers, and they get to use their martial arts die in place of the "normal damage" of the weapon. Other than that, the missile has all of its normal effects, since nothing about the monk's Deflect Missiles ability says otherwise.

So, the main ambiguity here is: what is the "normal damage" of the spine? I believe we can conclude that it is the d4. The damage is listed as "1d4 + 2 piercing damage plus 1d6 fire damage". This phrasing indicates that the piercing damage is the weapon's normal damage and the fire damage is "extra" damage added on top of that normal damage. The fact that the static modifier (which matches the devil's dexterity modifier) is applied to the piercing damage is further evidence of this conclusion. So, if the monk's dexterity modifier is +4 and their martial arts die is 1d6, then the damage of the spine when used by the monk in this way is 1d6 + 4 piercing damage plus 1d6 fire damage.

Drider longbow attack

Longbow. Ranged Weapon Attack: +6 to hit, range 150/600 ft., one target. Hit: 7 (1d8 + 3) piercing damage plus 4 (1d8) poison damage.

This attack has the same form as the tail spine attack, with the piercing damage being the "normal damage" and the poison damage being additional. The only difference here is that the normal damage die is 1d8, whereas our monk's martial arts die is a d6. That is no problem, however, since the monk is not required to use their martial arts die in place of the weapon's own damage die. Hence, the damage when the monk makes an attack with the drider's longbow arrow is 1d8 + 4 piercing damage plus 1d8 poison damage.

Sprite shortbow attack

Shortbow. Ranged Weapon Attack: +6 to hit, range 40/160 ft., one target. Hit: 1 piercing damage, and [additional non-damage effects]

Here, the confusion stems from the fact that there is no damage die. However, this is not a problem. Recall that a monk also uses their martial arts die to replace the normal damage of an unarmed strike, which is 1, just like the sprite's shortbow. If the monk can replace the 1 damage of an unarmed strike with their martial arts die, there's no reason the same shouldn't apply to any weapon with a normal damage of 1. Hence, the monk's attack with the sprite's shortbow arrow will deal 1d6 + 4 piercing damage. (Note that the monk could choose not to use their martial arts die and leave the damage at 1, but I believe their dexterity modifier would still be added to make it 5 damage total) In addition, nothing about the monk's Deflect Missiles ability takes away the additional effects of the attack, so the additional non-damage effects of the attack also apply to the target of the monk's attack (just like the additional damage effects in the previous 2 examples).

Summary

In summary, we had to parse a bit of English to figure out the "normal damage" of weapons with multiple damage rolls, and we used the example of unarmed strikes to conclude that the martial arts die can even replace the "normal damage" of a weapon with no damage roll at all. Once we did that, we simply applied the rules for Deflect Missiles and monk weapons in a straightforward manner to arrive at the result. In general, the missile, when used by the monk as a monk weapon, changes its damage die, attack and damage modifiers, and range, but it otherwise functions identically to the original attack.

Answer 3

In my opinion I believe it would be a judgment call on the particular projectile. If its a solid spike that is on fire, maybe the monk deflects or even redirects it, but maybe takes like a single point of fire damage or to the monks weapon, cuz hot is still hot, . But if it was an injection projectile, like a stinger that being shot, then the monk and deflect and redirect with out any trouble. I would judge in the moment or during preparation, especially if you have a monk player, or if you are the monk player, talk to your DM/GM, and work together on this. With this in mind, special equipment for a monks hands or weapons that would be beneficial or more susceptible to aid in the deflection/redirection of these special projectiles. And here is a thought on the spot, maybe even equipment that can aid in deflecting non-solid projectiles or even magic. Just a thought.