The Nightmare Plant is a plant introduced in the Ratboy's Nightmare update as part of gardening.
Nightmare Seeds can be purchased from Mr. Sims at varying prices. Whenever they are present in the shop, they will always be in the second slot. There are no other known ways to acquire these seeds.
When the plant is fully grown, it yields a single Fruit of the Nightmare which has no apparent purpose other than being sold for 20,000 gold.
With a guaranteed value of 20,000, however, players always profit from growing Nightmare Plants, and will gain 3,000 - 11,000 gold depending on the price they purchased the seeds at. With a steady supply of cheaply-bought seeds handy, money can be easily gained from these plants, though the 12 hour growth time will make harvesting them tricky for some players.