Potion, uncommon
This concoction looks, smells, and tastes like a potion of healing or other beneficial potion. However, it is actually poison masked by illusion magic. An identify spell reveals its true nature.
If you drink it, you take 3d6 poison damage, and you must succeed on a DC 13 Constitution saving throw or be poisoned. At the start of each of your turns while you are poisoned in this way, you take 3d6 poison damage. At the end of each of your turns, you can repeat the saving throw. On a successful save, the poison damage you take on your subsequent turns decreases by 1d6. The poison ends when the damage decreases to 0.
Notes: Consumable, Cursed
So you drink the poison, take 3d6 damage, make a CON Save, fail, and are poisoned. When you end your turn then, do you make a second CON Save? As in, are you making 2 CON Saves in 1 turn on the turn you drink this poison? Or do you just make the first save, and then (assuming you failed the first save), take 3d6 damage on the second turn, and then start making saves to reduce it?
You only make one save per turn. If you succeed on the first one, you're not poisoned and only take the initial 3d6 Poison Damage. If you fail the first save, then the situation might play out like this:
Drink the Potion and take 3d6 Poison Damage
Make Con Save: Fail
End Turn
Next Turn
Take 3d6 Poison Damage
Make Con Save: Succeed
End Turn
Next Turn
Take 2d6 Poison Damage
Make Con Save: Fail
End Turn
Next Turn
Take 2d6 Poison Damage
Make Con Save: Succeed
End Turn
Next Turn
Take 1d6 Poison Damage
Make Con Save: Succeed
End Turn
Next Turn
No Longer Poisoned
Best case scenario for if you drink this is 3d6 Poison Damage. Next best case (if you drink it and fail the first save) is 9d6 Poison Damage (3d6 + 3d6 + 2d6 + 1d6).
How would one use this on an enemy during combat? There are no rules for force-feeding, and, as anyone with a baby or pet would tell you, force-feeding liquids to someone who doesn't want them is a challenge! Command fails if the command is directly harmful (by the wording, they don't even have to KNOW it would be directly harmful). Would Charm Person work? Charm them, they become friendly to you, then you say "Here friend, drink this", then they drink it, then the Charm Person ends because you've done harm to them?
The way I interpret it is this: "This concoction looks, smells, and tastes like a potion of healing " stipulates look smell and taste. Which means that the illusion changes there and truesight would reveal that it doesn't look like a healing potion. Dispel magic could remove the illusion revealing all three. Alternatively you might decide that although it looks like it, certain properties of such are kept, which can be revealed with tool proficiences for instance. Like "Normal healing potions turn blue if you petal of cliffbloom to it. This potion's colour remained the same, it's a suspect."