The first AU that I have ever made! Brimstone!UnderFell
◤(THIS IS A LORE DUMP/IDEA AU DUMP, AND IS A WORK IN PROGRESS INDEFINITELY)
Brimstone is the name monsters give to the world beneath the surface, a place sealed away after the war between humans and monsters. The barrier still stands, separating Brimstone from the Above Ground, and the bitterness of that division has shaped everything that followed. The name itself is a joke that never stopped being funny because it never stopped hurting. Brimstone does not burn like hellfire, nor does it glow with sulfurous light. Instead, it grinds its inhabitants down slowly through cold, scarcity, and constant pressure. Survival here is not heroic. It is expected.
Life in Brimstone follows a strict, often violent order. Monsters are not mindlessly cruel, but kindness is dangerous when unprotected. Disputes are settled quickly, usually through strength or reputation, and those who cannot defend themselves learn to stay quiet. Laws exist, but they are enforced by fear rather than faith in justice. Most monsters keep their heads down, trade favors instead of trust, and learn which streets to avoid after dark. Death is common enough that it has become practical rather than tragic.
Snowdin is buried under ash-snow that never fully melts. The cold is harsher than it once was, and storms frequently spill into the Ruins and Catacombs. Heat is scarce, preserved only by torches and old systems barely kept alive. Beneath Snowdin, the Ruins stretch wide and quiet, not safe but less openly hostile. The Catacombs coil even deeper, stone-heavy and suffocating, a place where grief and paranoia fester. Hotland has merged into the central kingdom, forming a brutal industrial landscape of cracked stone, exposed machinery, and unstable heat vents that keep Brimstone from freezing completely.
Brimstone is ruled by the King, once the royal scientist under the Dreemurrs. After the death of the royal children and the withdrawal of Asgore and Toriel from power, he stepped into the vacuum without ceremony. The King governs through systems, laws, and detached oversight, leaving enforcement to the Knights of Blood. He does not care how order is maintained, only that it is. Rumors claim his work continues experiments begun long before the kingdom fell apart, though few understand what those experiments truly aim to accomplish.
The Knights of Blood serve as Brimstone’s enforcers. They are marked by enchanted gemstones embedded into their bodies, indicating rank and strength. The Knights are feared rather than respected, and most monsters avoid crossing them unless desperate. Undyne stands as their spear, relentless and uncompromising, believing that order is the only thing preventing total collapse. While the Knights maintain stability, they also deepen resentment, turning Brimstone into a place held together by pressure rather than unity.
The Dreemurr family remains fractured. Asgore lives alone in Old Home, tending dying gardens and memories he cannot abandon. Toriel hides in the Catacombs, her grief curdled into fear and suspicion. Their absence has left Brimstone without moral leadership, allowing brutality to replace compassion. The scars left by their children’s deaths still shape the kingdom’s silence.
Not all of Brimstone is uniformly dead or hostile. Beyond Waterfall lies Rainbow Fall, a strange and radiant anomaly that should not exist. It is sustained only through the presence of powerful SOUL energy, a truth hidden beneath its beauty. Lime, a synthetic skeleton created by the King using experimental monster soul remnants, tends the Falls. Though cheerful on the surface, Lime has killed before to keep Rainbow Fall alive, feeding it dust and blood. When threatened with its decay, he will do so again without hesitation.
Brimstone's weather is extreme, anywhere it was cold, wet, hot, it more extreme reaching other location, or new ones entirely., Whether this extreme weather is the result the war or the King’s ongoing experiments is unclear.
When Cyr falls into Brimstone, they disrupt a balance that was never healthy to begin with. Humans are not meant to survive here, and monsters know it. Some see Cyr as a threat, others as a resource, and a few as a mistake that should be corrected quickly. In a world built on endurance rather than hope, Cyr’s presence forces Brimstone to confront a question it has avoided since the barrier went up: whether survival alone is worth the cost of becoming something unrecognizable.
Sans carries himself with a lazy, mocking attitude, but the truth beneath his attitude is complicated and worn thin. He spends most of his time sleeping at his post or selling tacos in open violation of food laws, because he finds being a “proper” Knight boring and hollow. The toppings change depending on his mood, though he usually keeps the tacos tucked sideways into his jacket, causing the first one to fall out whenever he reaches for it. He likes vanilla in every form. He smokes vanilla cigars because the smell of ash reminds him of quieter days with his brother, and he drinks vanilla extract straight from the bottle. Sans talks too casually for a Knight of Blood, and when he first meets Cyr, he assumes they’re just another unlucky monster (due to his blindness in his left eye caused by the gem inserted in it) and offers them a taco without thinking. He enjoys teasing new people with crude or absurd jokes, especially when it makes them uncomfortable.
Underneath the humor, Sans is bitter and quick to anger. Mentioning Papyrus cuts straight through his jokes. Papyrus’s death and the brutal fight with Frisk hardened him, leaving him slow to trust and faster to snap than he used to be. Most of Sans’s worst traits come from grief, guilt, and the pressure of being one of the strongest Knights still standing. Despite that, he is fiercely protective of Togore Dreemurr (yes, the creature is in this au). Togore reminds him of Papyrus, not in appearance, but in their honesty and direct way of speaking. Sans often defends Togore when others mock their bluntness, and he listens to them more sincerely than he does anyone else.
Papyrus was not feared because he was vicious. He was feared because he was exact. As Lord of the Knights of Blood, he believed punishment should never be emotional, never impulsive, and never kind. He listened in silence, asked precise questions, and delivered consequences that fit the offense. Those who abused power were stripped of rank and sent into labor posts where survival depended on cooperation. Those who harmed civilians were marked publicly and reassigned to the coldest patrols until their reputation collapsed under exhaustion. Papyrus did not shout. He did not threaten. He simply enforced order with a calm certainty that made resistance feel pointless. Monsters whispered about him not because he enjoyed suffering, but because he made sure everyone remembered what they had done.
Outside of command, Papyrus was methodical and unexpectedly warm. He cooked rigatoni in vodka sauce with obsessive care, tasting constantly, adjusting heat and spice until it was right, and Sans loved it enough to steal leftovers without asking. The two of them worked well together, bickering without bitterness, trusting each other completely. Papyrus was always cold, always watching, but at home he loosened the scarf and talked about systems, fairness, and how Brimstone might survive without becoming something worse. He believed fear was a tool, not a goal, and that order meant nothing if it erased the people it was meant to protect.
Rosey The Flower
Rosey exists as a flower with no true SOUL, only a hollow space where it once was and as originally Chara. This emptiness grants them an unsettling clarity. They remember falling into Brimstone and dying there, and little else. Their thoughts move in sharp, efficient lines, stripped of warmth but not intelligence. Rosey is not playful, and they are not cruel. They are protective in a cold, utilitarian way.
When Cyr arrives, Rosey immediately recognizes them as a human who should not still be alive in Brimstone. To Rosey, survival here is not a miracle, it is prolonged suffering. They believe the kindest act is to end Cyr’s life quickly, before Brimstone grinds them down the way it did to everyone else. Chara watches the paths around the Ruins and listens to the hum of lingering magic. They appear without warning, rising silently from the earth. They dislike small talk, but will speak at length about survival, pain, and the nature of SOULs. Their voice shifts unpredictably, sometimes soft, sometimes sharp enough to chill the air. Their goal is simple: spare Cyr from what they believe is inevitable.
Togore Dreemurr
Togore is blunt, literal, and direct in a way that often unsettles other monsters. They struggle with sarcasm, subtext, and social games, preferring honesty over politeness. Togore asks questions plainly, even when those questions are uncomfortable or personal. This makes them seem fearless, though it is simply how they are. They find comfort in routines, patterns, and predictable environments. Togore enjoys dismantling machines, restoring old Dreemurr technology, and studying the gemstones embedded in the Knights of Blood, which makes the fact that his parents forget that he exist half the time even sadder.
They spend most of their free time in Old Home or wandering the edges of the kingdom, looking for anything that catches their interest. Togore is fascinated by Cyr because humans move, think, and emote differently from monsters. Their curiosity is intense but never invasive. They ask questions to understand, not to accuse. Their sincerity eases Sans’s temper, and the two form one of the few genuinely healthy bonds left in Brimstone. Togore does not fear Cyr or Chara. They simply want answers, why Cyr fell, and how a flower without a SOUL can still think.
Undyne, Lord of the Knights of Blood
Undyne is relentless, disciplined, and fiercely loyal to the King. She trains constantly in the Ruins, pushing the Knights of Blood to their limits through sheer force of will. Everything is a battle to her, combat, conversation, even disagreements over authority. She takes pride in her position and often patrols the old borders alone to test herself. In her rare moments of rest, she spars with enchanted training dummies or challenges fellow Knights to duels.
Undyne despises laziness and frequently scolds Sans, though she knows his strength is irreplaceable. She believes in order above comfort and stability above kindness. While she does not openly question the King, she remembers fragments of the world before things broke, and she buries those memories deep. Brimstone needs a spear, and Undyne has accepted that role fully.
The King (W. D. Gaster)
The King was once the royal scientist under the Dreemurrs. After the death of their children and the withdrawal of the royal family, he quietly assumed control. He rules Brimstone through law, systems, and fear rather than spectacle. He is not openly cruel, but utterly detached, consumed by unfinished work and theories that never reach conclusions. He speaks in fractured sentences and ideas that drift mid-thought, as though his mind is always elsewhere.
The King believes the barrier can be broken, not for peace, but for understanding. His research is built on experiments no one else fully understands. He takes immediate interest in Cyr, convinced that their SOUL represents a variable Brimstone has never accounted for. He does not rest. He calculates, writes, and tests endlessly, convinced that only he can impose meaning on a world that has rotted without it.
Asgore Dreemurr
Asgore lives alone in his abandoned castle in Old Home. He rarely leaves and avoids speaking to others. The deaths of his children destroyed his ability to lead. He still tends his gardens, even though the plants struggle under Brimstone’s ash-filled air. Asgore is gentle, apologetic, and hollowed by grief. Monsters treat him with a mix of respect and unease, unsure whether the former king will ever truly return to himself. He does not learn of Cyr’s existence until much later, and the knowledge shakes him deeply.
Toriel
Toriel hides deep within the Catacombs beneath the Ruins. Years of isolation have twisted her grief into fear and suspicion. When she first sees Cyr, she believes they are a hallucination and attacks in a panic, convinced her mind is finally breaking. Toriel is no longer gentle. She is brittle, frightened, and unpredictable, though traces of her protective instincts remain. She collects old books, candles, and dried flowers from her past life. Toriel is not cruel, she is shattered, and Cyr’s presence forces her to face wounds she never allowed to heal.
Grilbe
Grilbe runs a bar in Snowdin where monsters go to forget. His body is made of dense blue fire, with a glass jar for a head containing a green flame. He wears glasses and a light purple and black suit with a white tie. He speaks rarely, but everyone respects him. Beneath the bar, Grilbe operates an illegal food distribution network, passing coded notes and moving supplies for monsters who can’t afford official rations. In his free time, he experiments with recipes, trying to recreate flavors from before the war. Grilbe is calm, observant, and one of the first to sense that Cyr does not belong.
Lime
Lime(Based off the font Limelight) is a skeleton who wears a pumpkin over his head. Whether it is a mask or his actual skull is unknown, and he refuses to explain. He was created synthetically by Gaster using the soul of an experimental monster, part construct, part living essence. He lives near Rainbow Fall, tending its glowing plants and moss, which require a powerful soul to survive.
Beneath his gourd, lies a dark obsession. Lime has a taste for dust and blood, and he has killed before to feed Rainbow Fall, ensuring it stays alive. When Cyr arrives, he initially appears friendly, but quickly snaps, attempting to capture and kill them to use their SOUL. His dual nature makes him unpredictable.
Aleric
Aleric is a bat-like monster inspired by old vampiric myths. He has sharp red eyes and a voice that echoes softly when he speaks. He oversees Snowdin’s expanded library, which stretches deep underground into forgotten halls. Aleric is elegant, curious, and deeply protective of knowledge. He studies ancient texts about Brimstone’s origins, the war, and the Dreemurrs. Polite but intimidating, he becomes fascinated with Cyr upon learning their SOUL resonates with curiosity. To Aleric, Cyr is not a threat, but a question Brimstone has never been able to answer.
Cyr
Cyr is the fifth human to fall into Brimstone. They have long blonde hair that hangs over their eyes. They wear a light blue sweater with a single white stripe at the end of each sleeve, already worn thin by the cold. Cyr moves cautiously, curiously, watching how monsters react before deciding how to respond.
Their SOUL is orange, the trait of curiosity. It pulls them forward even when retreat would be safer. This SOUL grants Cyr nine lives. Each death reminds Cyr that they will run out if they keep being to brash, when the ninth is spent, the entire run resets. Brimstone forgets, though, three monsters remember something.
After a genocide run, Cyr begins hearing Frisk’s voice. It urges efficiency, repetition, completion. Frisk frames violence as learning, as optimization, as curiosity taken to its most brutal conclusion. It up to the player if they want to resist. (The player attempting pacifist and attempting to spare everyone is up to the player.
Muffet
Muffet thrives in Brimstone’s scarcity. She runs a web of food, silk, and information, selling survival at inflated prices and calling it generosity. Her spiders control trade routes through the Ruins and Snowdin, and crossing her usually means disappearing quietly. Muffet treats Cyr politely, even warmly, but only as long as they are profitable. Curiosity interests her. Resetting does not. If she ever learns Cyr can come back from death, she will try to monetize it.
Mettaton
Mettaton is no longer entertainment. He is infrastructure. His body has been rebuilt into a broadcast and control system used to relay Knight orders, emergency warnings, and propaganda. He performs only during rare morale events, turning fear into spectacle. Mettaton is charismatic, sharp, and deeply aware that he is being used. Cyr fascinates him because they introduce unpredictability into a system designed to suppress it. He frames danger as drama, but watches Cyr closely for signs of collapse.
Alphys
Alphys works under the King, maintaining failing systems and half understood experiments. She is brilliant, anxious, and drowning in guilt over things she helped create but no longer understands. Alphys recognizes Cyr’s SOUL as wrong almost immediately, not evil, but unstable. She wants to help Cyr survive, then realizes survival might make things worse. The possibility of resets terrifies her. Not because of death, but because it means mistakes can be repeated forever.
Napstablook
Napstablook drifts through the Ruins and Catacombs, untouched by most of Brimstone’s violence. They feel everything, but do nothing about it. Cyr does not scare them. They simply recognize the sadness clinging to them. If Cyr dies nearby, Napstablook lingers longer than usual, listening to echoes that should not exist.
Temmie
Temmie is a timid but determined business monster who survives by selling small handmade goods and scavenging supplies from a hidden shop tucked between collapsing tunnels. She avoids conflict at all costs, stuttering through negotiations and apologizing even when she is being cheated. Her prices are fair, sometimes too fair, and she often throws in extras out of nervous habit.
(Look at my repost for a bit more info)