Bottled Abyss (Patreon)

Potion, very rare

This small vial has a single dose of a fuming, pitch-black poison inside it. When ingested, the creature takes 4d6 psychic damage and must make a DC 18 Constitution saving throw. On a failed save, the poison takes hold. While the poison is in effect, the creature is blind and unable to sleep. When an affected creature attempts to rest, their dreams are plagued with terrible nightmares and are unable to gain the benefits of a long rest. The creature can reattempt the saving throw after 24 hours have passed.

The poison lasts until the affected creature succeeds on a saving throw or is cured by the greater restoration, heal, or wish spell.

Used by viscounts against kings, spies against kings, and queens against kings. And, of course, the butler did it.