A creature touches the Death Mist Vial, and speaks the command phrase or word as an Action to activate or deactivate it. It requires a DC 18 Arcana or Alchemy check to create the Vial, requires 200 GP in addition to the material components, a 6th level spell slot, & usually takes about 10 days of work to create a Death Mist Vial. Once the Vial is activated 1,000ft around it is covered in an obscuring black mist called the Death Mist. Undead within the Death Mist regain 3 HP at the start of their turn. Living creatures within Death Mist make a Con save when they 1st enter the mist each Dusk or finish a Long Rest inside it to determine some of the Death Mists Effects:

4d12 Necrotic damage that reduces Max HP, half as much on a successful save. The lost HP is restored after a Long Rest. Creatures that failed the save don't regain the HP until they pass the save, then complete a long rest. 

Disadvantage on Death saving throws regardless of passed or failed save.

Lower Light within the area by 1 value (Bright, Normal, Low-light, Darkness) regardless of passed or failed save.

If a creature is at 0 HP & unconscious at the start of their turn they turn into a zombie regardless of passed or failed save.

They cannot take reactions if they failed the save.