Nightshade

Deadly nightshade is one of the most toxic plants. While the roots are the most deadly part, the poisonous alkaloids run through the entirety of the plant. Scopolamine and hyoscyamine are among these toxins, both of which cause delirium and hallucinations. Consuming will offer you 10 Poison Damage. To gather it you need a Sickle.

Source
Original description: Found in deep shade of forests, picked on FULL moons, else poisons the picker.

Nightshade is generally found in outdoor areas near undead. Note that while nightshade plants can be found in the Plains Road Encounter, the plants are only ripe in the skeleton variation of that scene.

Currently, harvesting nightshade during the day will only result in one unit per harvest, but harvesting in nighttime hours (Evening through Late Night or :50 to :15 during Earth hours) will result in up to 10 units per harvest.

Trivia

 * Nightshade is a traditional reagent in Ultima.