Dungeon Master: Host's Poison

There are many different tools available to assassins. One particularly noteworthy tool is a strange necklace I was able to study from an undisclosed source. The necklace in particular didn't seem to be that unique in terms of appearance except for a small valve like feature that released water, despite no water ever being poured into the necklace. I was warned to be very careful about not ingesting it. A slow death would follow if I did.

Description

The design of the item doesn't matter. Any item can be used for this purpose, though jeweler and other non-suspicious items are most common. Regardless, there is some kind of hidden compartment in the item. This hidden compartment is filled either with water or a powder (nicknamed host poison though it is magical in nature). A wearer of the amulet is immune to all effects of the contents of the item of equal strength or weaker. When ingested, it acts as a curse that will kill that creature within 24 hours (the strength of the creature of the amulet determines the level of spell needed to remove the curse). Slight symptoms appear after 2d4 hours of being ingested. Such an item is typically used to treat someone with poisoned food while the assassin also eats the same food in order to try and deflect suspicion.

D&D 5^th Edition Rules

The strength of the item ranges from 1-9, corresponding to the level of spell used in the construction (3 is most common). The powder or water is only created from the item once a day but can be stored for a week (it evaporates, is corrupted from the humidity, or the magic wears off after 2 weeks). After 2d4, any creature that was not wearing a “host's poison” item of equal strength starts to feel slightly off. The effect tends to be minimum until the target finally drops dead after 24 hours.

Variant: Instead of acting as a curse, a poisonous powder or liquid seeps from the hidden compartment (it is created magically, but not magical itself). Someone with experience in poisons will be able to make an antidote (acts like applying a dispel magic spell but requires 50GP * level of the item to create).

False Legends?

Some have expressed the idea that such items don't really exist and instead came about from the legend of other items. Items that allow immunity to poison and containing a secret compartment for poison would allow the same strategies to be used. In order to match the legend perfectly, the poison used would need to be slow acting.

In-game: Grants immunity to poison and contains space to store 1 dose of poison inside a secret compartment (other liquids or powders can also be stored).