My players are hunting a group of vampires and have taken one down to 0 HP. In his misty form, how is damage tracked? He is at 0 HP, but not unconscious as to provoke death saves.
There's no need to track damage. HP can't go below 0, so (with a single exception, upcoming) taking more damage at this point does absolutely nothing. The only thing you need to know is whether the vampire takes more damage in a single instance than its max HP, which would kill it outright from massive damage.
Usually, when a vampire invokes Misty Escape, it's just out of the scene (or the scene is over, if there's no other point to it), unless the players have some means of turning it into a chase (unlikely, but possible).
My players are hunting a group of vampires and have taken one down to 0 HP. In his misty form, how is damage tracked? He is at 0 HP, but not unconscious as to provoke death saves.
Death saves are not tied to the unconscious condition, they are tied to having 0 HP.
The vampire is immune to non-magic damage, but can still be killed if it doesn't escape quickly. A vampire's lair will have plenty of ventilation gaps for it to move through walls to get to it's resting place without being attacked or followed.
Usually they either kill it quickly or fail entirely.
Best way to fight one is to use sunlight against it. Cannot even use Misty Escape if it is in sunlight. 5th level spell Dawn and 6th level spell Sunbeam do this.Also the 2nd level spell Moonbeam, while it does NOT count as sunlight, has the following "A shapechanger makes its saving throw with disadvantage. If it fails, it also instantly reverts to its original form and can't assume a different form until it leaves the spell's light." Vampires count as shapechangers, "Medium Undead (Shapechanger), "
It is not tracking HP, but time. Two hours to get to a resting place or it is destroyed. Typically a vampire is never further than 2 hours from it's resting place. That is 4 1/2 miles. Assuming it is inside that distance, it should be nearly impossible for the players to stop the vampire or beat it there. There is no rule that doing any damage to it in this form kills it. You might house rule of course. So there is no need to 'track' any damage yet.
About the only way to stop the the vampire for more than a couple of minutes without using high level magic is to use the spell Magic Circle. It lasts for one hour, which, if you cast it twice, should be able to keep the vampire from reaching it's resting place. But it has a casting time of 1 minute, so typically you have to use another spell to stop it.
However, you can totally kill it with sunlight, which is the intended method.
Assuming it gets to it's location (as it should), then it is paralyzed for one hour. You can attempt to move it without doing damage, bringing it to an area with sunlight. This will kill it. But with no stake, there is a risk: if it takes any damage before it gets to the sunlight, it reverts to mist form and escapes again. So instead, once it is in it's resting place you should first pound a wooden stake through it's heart. This paralyzes until the stake is removed. Possibly for thousands of years. So at that point minor damage should not revert it to mist form.
To truly destroy it, you must then take that vampire (ideally with a stake through it's heart) and move it into Sunlight. Or use a spell such as Dawn, Sunbeam, Moonbeam. (Note, Moonbeam stops it from transforming into Mist and if it cannot transform, it dies.)
Technically the only way to really kill it is with sunlight or those spells I mentioned.
Anyone hunting a CR 13 Vampire needs at the very least, a wooden stake. But honestly the ability to cast 2nd level Moon Beam , 3rd level Magic Circle, 5th: Dawn, or 7th: Sunbeam, will make it much easier to do.
My players are hunting a group of vampires and have taken one down to 0 HP. In his misty form, how is damage tracked? He is at 0 HP, but not unconscious as to provoke death saves.
There's no need to track damage. HP can't go below 0, so (with a single exception, upcoming) taking more damage at this point does absolutely nothing. The only thing you need to know is whether the vampire takes more damage in a single instance than its max HP, which would kill it outright from massive damage.
Usually, when a vampire invokes Misty Escape, it's just out of the scene (or the scene is over, if there's no other point to it), unless the players have some means of turning it into a chase (unlikely, but possible).
Death saves are not tied to the unconscious condition, they are tied to having 0 HP.
The vampire is immune to non-magic damage, but can still be killed if it doesn't escape quickly. A vampire's lair will have plenty of ventilation gaps for it to move through walls to get to it's resting place without being attacked or followed.
Usually they either kill it quickly or fail entirely.
Best way to fight one is to use sunlight against it. Cannot even use Misty Escape if it is in sunlight. 5th level spell Dawn and 6th level spell Sunbeam do this.Also the 2nd level spell Moonbeam, while it does NOT count as sunlight, has the following "A shapechanger makes its saving throw with disadvantage. If it fails, it also instantly reverts to its original form and can't assume a different form until it leaves the spell's light." Vampires count as shapechangers, "Medium Undead (Shapechanger), "
The question is how to track the damage? How much can the misted vampire take from the magical famage.?
I'd use death saves. 3 hits, dead. That is why a smart vampire will have its lair full of escape routes.
It is not tracking HP, but time. Two hours to get to a resting place or it is destroyed. Typically a vampire is never further than 2 hours from it's resting place. That is 4 1/2 miles. Assuming it is inside that distance, it should be nearly impossible for the players to stop the vampire or beat it there. There is no rule that doing any damage to it in this form kills it. You might house rule of course. So there is no need to 'track' any damage yet.
About the only way to stop the the vampire for more than a couple of minutes without using high level magic is to use the spell Magic Circle. It lasts for one hour, which, if you cast it twice, should be able to keep the vampire from reaching it's resting place. But it has a casting time of 1 minute, so typically you have to use another spell to stop it.
However, you can totally kill it with sunlight, which is the intended method.
Assuming it gets to it's location (as it should), then it is paralyzed for one hour. You can attempt to move it without doing damage, bringing it to an area with sunlight. This will kill it. But with no stake, there is a risk: if it takes any damage before it gets to the sunlight, it reverts to mist form and escapes again. So instead, once it is in it's resting place you should first pound a wooden stake through it's heart. This paralyzes until the stake is removed. Possibly for thousands of years. So at that point minor damage should not revert it to mist form.
To truly destroy it, you must then take that vampire (ideally with a stake through it's heart) and move it into Sunlight. Or use a spell such as Dawn, Sunbeam, Moonbeam. (Note, Moonbeam stops it from transforming into Mist and if it cannot transform, it dies.)
Technically the only way to really kill it is with sunlight or those spells I mentioned.
Anyone hunting a CR 13 Vampire needs at the very least, a wooden stake. But honestly the ability to cast 2nd level Moon Beam , 3rd level Magic Circle, 5th: Dawn, or 7th: Sunbeam, will make it much easier to do.