Dark Fountains are major plot devices in Deltarune. They are towering geysers of darkness that spew endlessly into the sky, their power transforming the area around it into a Dark World and shaping the objects within into Darkners that reflect its will. A Dark Fountain can only be created when a Lightner (whether human or monster) concentrates their desires into a sharp object and stabs the ground. If too many Fountains are opened at once, the sheer amount of darkness would initiate an apocalyptic event known as the Roaring, where Titans are born from the Fountains and bring ruin to the Light and Dark Worlds, all Darkners petrify one by one, and Lightners are left to survive in an endless dark.

As Darkners reflect the will of their Fountains, they can only truly "belong" in one Dark World. If they cannot find some kind of purpose in another Dark World, they turn to stone. However, the Grand Fountain in Castle Town is comprised of pure darkness, and can thus indefinitely support any Darkner that is recruited to live there.