After being decapitated by a vorpal dragon, the head can live on if the body is killed, but not vice versa. How would this play out?
The head and body seem to still be treated as one single creature, with one HP pool, AC, Fort, etc. So does it depend on which one the killing blow is applied to? Like, if the creature is dying 3, a blow specifically targeting the head would kill both, but a blow to the body would kill only the body?
If I understand it correctly, you question is "How do the mechanics work out if the body of the head is killed?"
Or is it more "what are the mechanics of attacking the head? And how much HP does each part have?"
I would say saves are the same...just for simplicity sake. Maybe make the body immune to mind affecting stuff. If you wanted to get more realistic there should probably be an additional dex penalty to the head, though you could argue the clumsy condition applied by Vorpal Binding is sufficient.
As for how much HP....honestly I don't know. I'd probably rule that basically any strike from the dragon is enough to kill the head.
Honestly, it doesn't make a ton of sense that the decapitation doesn't just outright kill.
Other than the fact that a dragon capable of making multiple attacks each round with a vorpal effect would be insanely hard to beat if a single failed save killed you outright. Consider that players are likely to have a fort save in the low to mid 20s against a DC 33. You've got a 50% chance (roughly) to succeed or fail. If that mean straight death it's a problem.
And with Heads will Roll it can do it against 2 PCs. And potentially can do it to a 3rd with it's remaining action (though it's probably wiser to fly away from the party).
Played very intelligently, the vorpal dragon likely targets and kills the heads if they're worried about surviving their assailants.
But it is a very good question how much HP the head should have.
If really forced to make a ruling, I'd probably just say the head takes like 10% of the HP of the creature at the time of bifurcation and body 90%. And say that each has the same number of wounds, if any, as the whole creature did before.
Fairness or lack thereof aside, I don't think the head should be trivial to destroy. If it is, then these fleshed out mechanics for the head controlling the body and what conditions the target has for what purposes would be mostly irrelevant.
I actually agree to an extent...
But I'm also confused why the monster has a vorpal attack that doesn't simply kill. Although the ancient vorpal dragon and vorpal archdragon can choose to have it kill the target, so it's really only the young and adult version of the vorpal dragon. Which I suppose it because the devs didn't want low level threats to be so...threatening.
I haven't done the math, but I feel like the chance to be hit by the chin blade attack and the subsequent save honestly ends up with approximately the same chances of beheading. So I'm not sure why they let it happen at level 18, but not 13.
If you wanted it to be less trivial to destroy the head, you could make the head and body share the HP pool. I don't particularly find that "realistic" but also maybe it doesn't need to be.
But then if they shared a pool...both would die at the same time...
Well I suppose that would only happen if they shared wounds too.
Anyways...the mechanics could use a bit more explanation.
Hitting with the chin isn't enough. The dragon needs a nat 20 to force the Fort save vs beheading.
So take the chance of failing that Fort save, and divide it by 20. That's the chance of a decapitation.
Good point about the nat 20 needed.
That's not even a crit, but a true nat 20.
It's honestly not even going to come up very often.
Honestly the most likely thing to happen here is the dragon starts of combat with Heads will Roll, and afterwards flies up and way from the PCs to get out of range of most things. Possibly staying around 90ft away so it can use it's pinion attack against the PCs while also maintaining a decent range away. Ultimately it should probably keep it's distance and wait until it can use Heads will Roll again and just keep slowly wearing down the party while staying mostly out of the range of reprisal.
It is very strange that the head can live without the body. The head is still a living creature (I think?), so it should need to breathe and eat, but it has no lungs or stomach. Does the vorpal dragon even feed the heads that it keeps as trophies?
It's also strange that it doesn't work both ways. Why can the head survive without the oxygenated blood and nutrients from the body, but the body can't survive without the brain signals, air, and food from the head?
As for mechanics, I'll probably run it like a Summoner and its eidolon. Sharing stats, but each part being able to be targeted separately. Aoe attacks hitting both individually though. If you go to 0 due to an attack on the body, your head is fine, if you get to 0 due to an attack on the head, you die.
I don't think it should be easier to wombo combo the decapitated creature with AoEs than a summoner. Even the summoner and eidolon only take the worse result between them, and they are literally two creatures. The decapitated creature is presumably still one creature, and that one creature is already clumsy/dazzled/blinded/deafened. I also don't see why the head and body would double up on damage from a fireball while separate when they didn't do so while they were attached.
I think it's best to treat the creature as simply occupying the body's space in addition to the head's space (probably Tiny if the creature was Medium or smaller), maybe using only one or the other to determine where to start calculating range and line of effect for certain actions (bow Strikes fired from the body, Demoralize spoken from the head, etc.).
If the creature holds its own head, then it's all just one creature in one Medium space. Someone Striking that space can specify whether they aim for the head or body (kinda like fighting a hydra), but it only matters for the killing blow.
If the creature suffers a killing blow, the head survives if only the body is targeted.
If the creature suffers a killing blow, the head survives if only the body is targeted.
I suppose ultimately that's the most fair way to run it. The head and body share their HP pool, saves, etc. And if the dragon wants to really kill you they target your head. If they want your head as a trophy, they attack the body. Of course...it does raise serious question about what does the head do for sustenance.
While the concept is interesting....there's a lot of stuff left open that doesn't really matter if the party wins, the dragon is defeated, and anyone that was bifurcated can reattach their head. However, if someone need days or weeks to survive and their body was destroyed...lotta question. A head has no lungs, no digestive system, no place to store energy (fat).