This page refers to the original Nightfall, or the one Vespera built.
It was located beneath the gold statue of an elf and human at Atlantis, which is somewhat ironic due to the fact that the statue depicts friendship, while the laboratory underneath depicts a far more horrifying image. It was created by Vespera, an extremely powerful Empath whose emotions became numb, so she could not feel guilt. She experimented on humans to see how they could do horrible things without breaking their minds. This is later recognized as why the humans plotted to overthrow the elves. Her Nightfall later inspired Gisela's, which had the same purpose, however, it was built in a different place that was only unlocked with Keefe's blood. This Nightfall has the symbol (her signature) carved onto the statue to mark it.
The inside is covered in mirrors that can project the same image. It serves to confuse intruders and to taunt them. There are also multiple rooms and levels for the occupants. Although this was a place that caused much strife for humans, it was essential in the plot of Nightfall. It was also later destroyed.