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?
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?
Best Answer
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.
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.