The Nightmare Plant is one of the eight plants that can be cultivated and harvested by the players from gardening, a feature introduced during the Ratboy's Nightmare Update.
The Nightmare Plant can appear after a Nightmare Seed has been cultivated and grown over the course of 12 real-life hours. The Nightmare Seeds can be purchased from Mr. Sims, at a price range of 9,000-17,000 gold. It can also be obtained as a 'rare' drop from mobs of the Nightmare.
Once the players harvest the Nightmare Plant, it will yield a single Fruit of Nightmares that does not have any apparent usages other than to be sold to a selling vendor at a selling price of 20,000 gold.