xarph asks: How does corruption work? Are other members of VM succeptible from proximity or spending time in Whitestone?
Here’s the TL;DR version: No one is really sure what “One point of corruption” means. There are a lot of theories (a few we’ll discuss), but it’s not canon to 5e. Percy’s class and backstory were born under Pathfinder, as were a few other mechanics, so it’s likely Pathfinder is the source of the Corruption mechanics.
That’s the summary of what follows. Here’s our breakdown.
It’s obvious that Whitestone is under a curse of sorts. Plants can’t grow in the town proper, undead roam the streets, and SOMETHING is happening under the castle. 5e does have its share of curses. I was immediately reminded of the Curse of the Rakshasa, in which victims of its claw attacks are beset by nightmares whenever they attempt a short or long rest. The victim cannot regain their HP or used spell slots, and gains a level of exhaustion. The curse can be removed by Remove Curse or Greater Restoration.
However, Corruption is not Exhaustion, as far as we know. We believe that all of Vox Machina is susceptible to this curse so long as they stay in the city. It is mighty convenient that the only one who fell to the curse was the character whose alignment we already question and who we assume made a pact with an other-worldly fiend of some type. However, considering the effect on the rest of the town, we would be surprised if more members of Vox Machina don’t fall to it the next night, as well.
In our research on Pathfinder’s corruption, we found this article on damnation. It very much matches what we can speculate about Percy so far, but not so much the rest of the party. Corruption can also be used to heal undead while dealing damage to the living. It’s possible that the system is entirely of Matt’s creation. We’ll have to wait and see if there are more effects than waking up with a sore throat the next day.
Update 2: gilded–knight points out that 5e’s madness system could also apply. (DMG, p259). If Orcus’s second horn is in play, the god of undeath could be responsible for the regional effects, resulting in madness. While madness is a binary effect (you’re either mad or you’re not) and there are not any examples that affect physical health, there is no reason to believe that Matt wasn’t inspired by it.