The Nightmare Realm(dimension O) is a seething, intergalactic foam between dimensions where Bill Cipher resides in after destroying his home universe. He sought to escape by entering the world of Gravity Falls, Oregon in an event known as Weirdmageddon.
At least 1 trillion years before the series, Bill Cipher "liberated" his home dimension by burning it and the residents there, then proceeded to take over the empty Nightmare Realm as his own. He described the Nightmare Realm as "[his] own decaying dimension" in which he had "been trapped in for one trillion years" looking for a means to escape, as its lack of constant physics doomed it to self-destruct in the future. When Stan finally activated the Universe portal in an attempt to save his brother Ford, it creates an Interdimensional rift. Bill later uses this to create a gateway between Dimension 46 (referring where Gravity Falls is) and the Nightmare Realm.
In "The Last Mabelcorn," Bill appears within Stanford's dreams to haunt him, and warn him of the upcoming inevitable events. While doing so, he gives Ford a glimpse of the Nightmare Realm by shattering the rift. Later, Bill, who is in the Realm, watches Ford and Dipper successfully establishing a shield around the Mystery Shack and starts to choose other people to resume his plan.
In "Dipper and Mabel vs. the Future," Bill has possessed the body of Blendin Blandin in order to retrieve the rift from Mabel Pines, which he uses to create the previously prophesied gateway to the Nightmare Realm.
In "Weirdmageddon Part 1," Bill uses the newly created portal in the sky to bring forth his gang of nightmare monsters and inter dimensional criminals from the Nightmare Realm.
In "Weirdmageddon 2: Escape From Reality," the Nightmare Realm is still visible through the tear in the sky above Gravity Falls created by Bill. However, the inter-dimensional criminals can't leave Gravity Falls, Oregon; according to Ford, natural law of weirdness magnetism of the city contains Bill's weirdness, and an equation can disable the barrier.
In "Weirdmageddon 3: Take Back The Falls," when Bill was destroyed through the Pines twins' cunning, his minions were sucked back into the Nightmare Realm, never to be seen again.
In "Gravity Falls: Lost Legends", the Nightmare Realm is the major settlement for the tale "Don't Dimention It!", Mabel falls accidentally through one of the remaining rifts created by the Rift, after the events of the finale, thanks to a Hand Monstrosity (previously mentioned in Journal 3). She manages to evade it, but falls through other dimentions, among them the place where Bill and his Henchmaniacs were imprisoned (we see Keyhole and 8 Ball once again staring anrgily to a statue of Bill, briefly), until reaching MAB-3L. Later in the tale Stan and Ford come to the Nightmare Realm looking for Mabel, until Ford decides to use a beacon to call a samaritan, who doesn't allow them to take a ride, due Ford's criminal record. So Stan persuades him to let them enter while aprehending Ford. The Nightmare Realm is seen once again after Anti-Mabel's defeat, when the other Mabels give Stan, Ford and Mabel a ride to the rift, before returning to their respective dimentions.
The Nightmare Realm, from what is shown, is a somewhat surreal unstable void filled with swirling shades of dark purple, red and yellow almost like a lava lamp. In the real-life canonical Journal 3, it says that the Realm is littered with asteroids and has a "smell of burnt hair."
In "Gravity Falls: Lost Legends," the Nightmare Realm looks like space, filled with stars and other celestial bodies and littered with Weirdness Bubbles. It seems that the Nightmare Realm is split in zones, because the zone where 8 Ball and Keyhole were (and subsequently all of Bill's Henchmaniacs are), resembled more the Nightmare Realm that appears in the series, described above.
The Nightmare Realm has been used as a home for some of the worst criminals of the Multiverse, including Bill and his Henchmaniacs (such as 8 Ball, Amorphous Shape, Hectorgon, Keyhole, Kryptos, Paci-Fire, Teeth, Pyronica, Lava Lamp Guy, The Horrifying Sweaty One-Armed Monstrosity, Creature with 88 different faces, and Zanthar) however, due to it connecting to many other universes, beings from more dimensions have found themselves stuck in the Nightmare Realm with no means of escape. These include a group of asteroid miners Ford encountered after first entering the Realm. In "Gravity Falls: Lost Legends," for what it seems the Nightmare Realm is split in different zones, the miners could had gotten stuck in another zone. Other inhabitants include a Chtulu offspring and the Hand Monstrosity.
- 215. "The Last Mabelcorn"
- 217. "Dipper and Mabel vs. the Future"
- 218. "Weirdmageddon Part 1"
- 219. "Weirdmageddon 2: Escape From Reality"
- 220. "Weirdmageddon 3: Take Back The Falls"
- In Gravity Falls: Lost Legends, the Nightmare Realm is referred as Dimension 0.