How to extract yuan-ti poison?

Question

Through a series of lucky Persuasion rolls, my party managed to get a yuan-ti malison on their side. The player of a recently deceased player character wanted to play as the yuan-ti, which I agreed to because it seemed like it would be fun.

However, he just messaged me to say he wanted to extract the 2d6 poison damage from his bite attack and use it to coat his weapon. This sounds like a really cool idea, but I don't know of any sort of system to employ for self-poison-extraction.

What system should I use for this self-extraction and application process?

Answer 1

AFAIK, there are no rules for that. We can extrapolate the rules given for harvesting from other creatures, though.

Some poisons from the DMG can be harvested

For example, the Serpent Venom (DMG p. 258)

This poison must be harvested from a dead or incapacitated giant poisonous snake.

Most of these poisons don't state "from a willing..." because they are harvested from creatures that aren't likely to be willing, IMHO. I don't see how adding "willing" to the text would break anything.

In this case, the damage done by the poison (3d6) is equal to the damage done by the poison from snake's attack, so keeping the 2d6 is consistent.

Rules for harvesting

A character can instead attempt to harvest poison from a poisonous creature, such as a snake, wyvern, or carrion crawler. The creature must be incapacitated or dead, and the harvesting requires 1d6 minutes followed by a DC 20 Intelligence (Nature) check. (Proficiency with the poisoner's kit applies to this check if the character doesn't have proficiency in Nature.)

I would say that, for self-extraction, the creature knows enough about himself to be proficient with knowledge about himself, so you always add the proficiency bonus. The DC might also be lower, but that's your call.

Note on duration

Per DMG Errata, Injury Poisons (which usually are the harvested ones) say

Injury. Injury poison can be applied to weapons, ammunition, trap components, and other objects that deal piercing or slashing damage and remains potent until delivered through a wound or washed off.

So while it's a fun idea, note that it is a one use only (per application). You can, obviously, restate that - but the reason this errata was added was because otherwise poison is broken. See Poison Bug (2014, pre-errata). In particular, Extra Attacks become insane.

Answer 2

Extracting venom from snakes is a common practice, often done to study the venom or produce an antivenom. You might find this link helpful, though you'd have to adjust some things considering it's a friendly yuan-ti and not a confused snake.

https://www.wikihow.com/Safely-Extract-and-Store-Snake-Venom

The player would likely need a waterproof container covered in thin fabric or a similar material, and then bite through the fabric to inject venom into the container. Once you have enough venom, you could coat your weapon in it.

Note that this would likely be done during a short or long rest - it would probably be difficult to do during combat.

However, if the venom is something the yuan-ti can spit or has in their spit, this would be a lot easier. They would just need to spit venom on the weapon.