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?
Best Answer
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)
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
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
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.