The End Well is a naturally occurring structure found in the Ancient Pig Ruins on the 5th island. It resembles a Wishing Well, but with Nightmare Fuel instead of water. It emits a sound that resembles the insanity ambiance.
Placing Nightmare Fuel into the End Well will cause it to spit out 1-3 purple gems. Placing any other item in the well will cause a Nightmare Creature spawn (extensive testing not done).
The player can use this to turn grass into an infinite amount of purple gems. By placing a commonly found item, like twigs, into the End Well, the player can then kill the Nightmare Creature that spawns and place the Nightmare Fuel into the End Well.