This encounter is best used when the PCs are traveling by road. There are two very different ways to kick things off, which will depend on whether the party triggers it by day or by night. As we all know, you cannot have a meaningful campaign if strict time records are not kept, so it may be helpful to keep in mind what time of day the party comes upon this encounter to play things accordingly. Or just fudge things so that the party happens upon it at the right time of day for the scenario that most interests you.
You may want to mark the location of this encounter on a hex or point map, if using one, to keep note of it in case the PCs revisit it. They may find new developments if they do so by both day and night!
Background
The village of Tedansk has a problem - everyone is dead. An alchemist by the name of Teodosija once lived a few miles away from town, where - like many of her kind - she devoted her studies to decoding the secrets of life itself. She sought the ultimate goal of any alchemist - the fabled Elixir of Life, which could grant her immortality. As part of the process or creating this elixir, Teodosija managed to create a Philosopher's Stone, and with that, she began a ritual that would allow the elixir to be distilled from a bit of the Stone mixed in wine.
The only problem was, Teodosija discovered a little too late that her Philosopher's Stone was unstable, and the ritual did not go as planned. When she dissolved the Stone, it set off a magical reaction that vaporized her instantly and sent a wave of force through the land, affecting Tedansk. No physical structures were damaged, but everyone in town immediately dropped dead on the spot. Teodosija did, however, manage to imbue the Stone with the power of life - but not quite as she intended.
Now, the entire town of Tedansk is cursed with undeath. By day, their bones lay moldering wherever they fell, but at night, the ghosts of the villagers arise, going about their lives as though nothing ever happened.
To further complicate matters, Teodosija had previously constructed a shield guardian to guard her tower against intruders, but shortly before the event, she came to town to announce her "successful" creation of a Philosopher's Stone - and in the process, the village's resident ne'er-do-well, Milenko, pickpocketed the amulet she used to control it, believing it to be nothing more than a gold necklace. He was later caught for an unrelated crime and placed in the town lock-up, but before the town council could figure out what to do with him, the wave struck. Teodosija never figured out what happened to her amulet before her death, either - and, as she never reported it missing, it went unnoticed when he was apprehended.
The Encounter
If the PCs come upon Tedansk during the day, they will find it eerily still and quiet, with no signs of life. The buildings are undamaged, but no one walks the streets; everyone, from shopkeepers at their desks to the children gathered around a hopscotch track in the town square to even the animals in the fields, have fallen dead and long since rotten away to skeletons, and everything is covered in dust and cobwebs. Even the trees surrounding the village are dead.
As soon as the sun sets, however, the skeletons and dead plants disappear, and in their place the spirits of the townspeople, their livestock, and the vegetation arise, with the people going about their day-to-day business as normal. They do not acknowledge the fact that it is night, or the amount of dust on everything, and they will interact with the PCs as any living person would. If the PCs say anything to them that references them being dead or ghosts, the townspeople will be confused and likely offended; pressing this line of questioning will make them run the party out of town as lunatics.
Tedansk consists of a few houses, a shrine to Solenna, a blacksmith's shop, and an inn. All of them operate as normal, though any food served by the inn will have long since gone rotten and withered away (the villagers do not notice this). In addition, there is a lock-up where Milenko is being kept. A guard stands outside and will explain that he is being held for the crime of theft (he stole a cask of ale from the inn). Milenko will deny having stolen Teodosija's amulet, but if coerced, will eventually admit that he did pickpocket a necklace from the alchemist, which he gave to his fiance, Savitza, two months ago. Savitza is also a ghost in the town; she does not know that Milenko stole the amulet, and will be alarmed (though not entirely surprised) if she learns the truth. Neither of them know of the amulet's true nature, and Savitza will be unwilling to part with the amulet - it was a gift from Milenko, after all, and even if he is a thief and a cad, she remains loyal to him nonetheless, as their love is true.
Rumors in town may lead the PCs in the direction of Teodosija's tower, which sits further back in the dead woods to the southwest, about half an hour from town by foot. The townspeople know her as an alchemist of great power. Many of them respect her knowledge, and even more fear it; about two months ago she came to announce her success in creating a Philosopher's Stone, but she hasn't been heard from since. Some suspect that she was destroyed by her creation, while others wonder if she's up to something more nefarious.
Further DevelopmentsIf the party decides to investigate Teodosija's tower, they will find it to be a squat two-story stone construction deep in the woods. The first floor is guarded by Teodosija's stone guardian, which will attack anyone it is unfamiliar with; however, it will not attack anyone wearing the amulet, and will respond to their commands. This floor also contains Teodosija's living quarters; nothing of value is to be found there.
A flight of stairs leads up to the second floor. The shield guardian will not let anyone up the stairs without a fight. However, a DC 15 Investigation check outside the tower will determine that there are enough loose stones and patches of moss along its walls to allow someone to climb up to the second floor and enter through a window. A DC 20 Athletics or Acrobatics check, or the use of rope, pitons, or other tools, is required to do this.
The second floor serves as Teodosija's laboratory and office, with many tools for brewing and distilling, jars of ingredients and reagents, the obligatory taxidermied alligator, and so on. On a table in the center of the room is set a large red stone - this is Teodosija's Philosopher's Stone. Nearby is a hammer and chisel and a mixing bowl in which some wine is placed, left over from her failed experiment. Teodosija does not have a ghost, as she was consumed in the center of the reaction; all that is left of her is some dust scattered about the floor.
Also placed here is Teodosija's journal, where she has recorded her notes and a log of her experiments. In it, she notes that she has mastered the Philosopher's Stone, and intends to use it to create the Elixir of Life by dissolving a piece of it in wine. She also discusses coming to town recently to reveal the news. In the next entry, she notes that she can't find the amulet for her shield guardian anywhere, but won't let it distract her from her progress. It is evident from a glance at her notes that Teodosija was a passionate and perhaps exuberant woman, but one who let herself get carried away in the pursuit of knowledge and didn't take proper precautions, and was easily distracted. Looking through previous entries will reveal that she often boasted of making a breakthrough on an experiment, only to curse her lack of foresight on some important matter the next day (for example, one report describes her brewing a potion, only for it to quickly evaporate because she left the burner under the flask running while she was doing something else).
In particular, a DC 15 Arcana check will be able to determine that according to what her notes describe, Teodosija did not properly perform the phase of multiplication. The crucible that would form the Philosopher's Stone should have been continuously fed with quicksilver; instead, she describes adding quicksilver at intermittent intervals, apparently feeling the proper procedure was too much work. This indicates that the Stone was not properly stabilized. A character with a background in alchemy will know this automatically upon reading this report.
Teodosija has enough alchemical equipment to comprise a set of alchemist's supplies, if one wishes to take them. However, there is too much in the laboratory for any one person to carry in a single trip. If one uses the complete set of equipment (which cannot easily be moved from the room) to perform alchemy, they will receive Advantage on all applicable rolls. Proficiency bonuses still apply.
Of course, the bold might be willing to take Teodosija's Philosopher's Stone. Any alchemist would be willing to pay a hefty price for such a treasure - though if they realize how volatile it is, they will not be happy, and may seek retribution against the PCs.
If the PCs wish to use it themselves, it can turn lead into gold if applied to it using a ritual that takes five hours to set up and perform, but due to the instability of the stone, there is a 75% chance that it will instead cause the lead to combust, destroying it. If a chunk of the stone is broken off and dissolved in wine, roll 1d100 for the effects:
1-10: The combination sets off another violent reaction. All living creatures in a 10-foot radius must make a DC 25 Constitution save or suffer the effects of a disintegrate spell. Constructs, undead, and inanimate objects aren't affected - this is specifically a reversal (as above, so below) of the Stone's effects on life. In addition, all living beings in a 90-foot radius around that must make a DC 25 Constitution save or drop dead instantly. They will rise as ghosts at nightfall, and will have no memories of dying.
11-50: The reaction produces a potion of poison.
51-90: Nothing happens.
91-100: The reaction produces the Elixir of Life, which will remove all disease and other negative status effects from anyone who drinks it, in addition to healing any injuries (including regrowing lost limbs or other body parts) and cause the person consuming it to become 1d6 years younger.