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For other versions of Underfell, see Underfell (disambiguation).

ArsonFell is an AU of Undertale and a Sub-AU of Underfell. In this AU, the Level of Violence is measured with a real physical element: fire. When someone in this universe gains LOVE, a patch of fire will appear on their body with each kill. If someone has too much LOVE, they are more prone to setting things on fire without thinking. The fire will grow or die down drastically depending on the two emotions; the fires on the person's body will flare up when angry, and die down when sad.

Most people that are left in the UNDERGROUND will have at least one patch of fire, as this will boost their immunity to being burned and breathe around smoke. The location of the fire will be determined by how they gained their LOVE and the amount of HP of who they killed. If someone were not to gain any LOVE, they will turn to ash between 2 months to 4 years. On the other hand, if someone were to gain too much LOVE, they will suffer combustion and will leave nothing but ash. Humans and monsters are included in this scenario if they are within MT. Ebott.


There was no part in history were humans and monsters lived in harmony. War was always raging. Some monsters and some humans did find a way to co-exist; however, that didn't stop the humans from forcing all of the monsters that were still alive into MT. Ebott.

Once in MT. Ebott, the monsters seemed fine with the state of things. For a while. They found food, water, they had shelter. It all seemed fine for a few years. But, then the first human fell into the UNDERGROUND. The monsters started to attack the human from unfiltered rage, but the son of the King and Queen stopped them before they did. The prince was still in his mother's womb when the monsters were thrown into the mountain, so he did not know humans like they did. Instead of hurting the human, the young prince took the human home with him while his father was busy with the royal scientist.

Once there, the son of the king and queen showed the human to his mother. He told her that the human needed help and a place to hide. The queen was at first shocked at the sight of the human child, but she merely smiled and nodded. The queen and prince gave the human fresh clothes, something to eat, and a place they could hide whenever the king returned.

It was like that for some time. The queen and prince treated the human like a member of the family. However one day, the king came to their home earlier than normal. The prince and the human child were drawing pictures in a sketchbook their mother had found. The queen was reading a book full of poetry. When the king returned home, they were not expecting him. They tried to hide the human, but they were too late. The king saw the child and in a fit of anger, started to raise his weapon.

The queen and the prince stopped the king and tried to tell him not to hurt the human. The king smacked his wife away and told the prince about the terrible things they have done to monsters, and that they needed the human's SOUL to get out of the UNDERGROUND. The prince still tried to sway his father's actions. But, to no avail. The king's face grew dark. He told the prince that if he valued the human more than monsters, he could die along with them. He shoved his son away, stabbing both his son and the human. Blood and dust-stained the floor of their home. Soon after that, the queen fled towards the RUINS with the body of the human and as much dust from her son as she could get. She did so before the king had a chance to take the SOUL. She later buried the human's body within the RUINS, scattering her son's dust in the same spot.

Two fires appeared on the king's body, one on the back of his head and the other on the side of his chest.

Soon after the murder of the prince, everything started to go to Hell. Monsters would kill each other until some more humans fell into the mountain. Most of the monsters wanted the human SOULs for themselves, but they were always forced to give the SOULs to the king. One monster in particular didn't agree with the king's methods and tried protecting one of the humans, the Green-SOUL Child.

When the king found out about that, he ordered the royal guards to infiltrate the monster's home. The monster felt he had no other options and apologized to the human before killing them swiftly, taking their SOUL before the king could. The monster was deemed a traitor and would have suffered bigger consequences if his brother didn't step in to help.

More time passed after that. Fire filled up the UNDERGROUND. Dust and blood tainted what was left of the shrubbery. The air was filled with ash and smoke. Monsters had depleted in size, most losing their minds and finding their own means of survival. Some monsters that still had their minds intact wondered if that was the humans' plan when they sealed them into the mountain.

For all of them to lose their minds, and kill each other off in their stead.



  • Frisk is an older teenager.
  • They feel safer alongside Sans, but only when he's not mad.
  • They often have a friendly nature that almost never mutual. They seem timid at first around others, but they are actually quite fearless when it comes to someone's safety. Frisk is patient but will become upset if monsters start being mean to each other.
  • If side by side, this Frisk is about a few inches shorter than Sans.
  • After getting out of the RUINS, they have a patch of fire on the back of their left shoulder.
  • Unlike many the other versions of Frisk, this Frisk cannot manually reset or load. Instead, they go through a phenomenon called Auto-Checkpoint Load (checkpoints replace the SAVE points). Meaning the only time they can reset or load is during this phenomenon, and this requires them to be killed... or kill themselves.
  • They once had 20 HP, but after the first ACL, they only have 1. Food doesn't seem to heal their HP.
  • If calculated, Frisk during the Neutral Pacifist Route would technically be LV 4. Only 4 because the fire they got was from a mercy-kill, which in point halved the EXP. This in return, kept their Murder Level at 0.
  • Side note: has a weakness for things that are small, cute, or dog-related.


  • Not much is known about Chara.
  • They appear to be in a limbo-like area with their adopted brother Asriel, though they can experience what Frisk does.


  • Never became Flowey.
  • In a sort of limbo with Chara.
  • His SOUL was preserved by Chara's for a long time.
  • His SOUL and Chara's SOUL fused with Frisk's when they landed in the UNDERGROUND.


  • Toriel makes only a brief appearance.
  • There was always a look of sadness on her face.
  • She was the only person left in the RUINS, save for burnt-out husks of a few Froggits and Whimsuns.
  • She had a patch of fire on her left shoulder and her left hand, indicating that she indeed gained LOVE.
  • How she gain LOVE was killing two children who fell before Frisk: the Blue Child and the Orange Child. Toriel was sure to give both a quick and painless death. She was tired of seeing children fall down into the mountain just to die from Asgore that she allowed Frisk to end her.
  • She wanted to die for years after the death of her son Asriel and her adopted human child Chara at the hands of Asgore.


  • Is 5 feet and 10 inches tall, about 9 inches shorter then Papyrus.
  • Sans is an emotional and nervous wreck that suffers from a case of PTSD, hiding this by either distancing himself from others or making jokes (dirty ones very much included).
  • He isolated himself from the other monsters after some monsters started killing each other, and after an incident regarding his brother, Undyne, and Asgore.
  • He gained two of four fire patches when he killed two monsters to protect himself and his brother Papyrus when they were younger, and then mercy killed the third fallen human when he was older. The fourth fire patch was gain when he murdered Undyne. Sans believes that he had a justified reason to do it, and the scarf around his neck is proof of that.
  • He absolutely hates Alphys, and constantly harasses her about Undyne and makes fish jokes whenever he is around her.
  • Sans' temper is very short when people talk about Papyrus or try to touch Papyrus' scarf, evening breaking a monster's hand when they tugged on it.
  • He appears to have a soft spot for Frisk, but he doesn't know the reason. This causes him to be murderously protective of them.
  • Absorbed the KINDNESS child's SOUL.
  • It is said that he is stronger than most of the other Sanses of the multiverse, though not by much.
  • Side note: Whenever AUs collide, he gets very uncomfortable around Fresh!Sans.


  • Papyrus was a very well-known sentry in Snowdin.
  • Fame and fortune did not rest within this skeleton's eyes, but he did like receiving attention.
  • He loved his brother Sans dearly, but he would make them both act like they didn't have anything to do with each other and would ask his brother to refer to him as "boss" when around others.
  • Papyrus often took beatings for his brother when he was alone with Undyne, then would tell Sans the bruises were from falling. Though he knew his brother was too smart to be fooled by a lie like that.
  • He was killed when he refused to end Sans' life, disobeying orders for the first time.


  • Grillby is a manipulative glutton.
  • It is said that the fire on his arm was the first one that he gained. No one alive knows for sure.
  • One might mistake the fire on his belly for just one, but in reality, it's a big clump of several fire patches.
  • The flame monster has an unusual liking to the "snow" on the ground of Snowdin, even going to the lengths of making snow cones and sharing them with any passerby.

Monster Kid

  • Monster Kid is not aggressive, at times.
  • Their personality is different when talking to Sans than when talking to Frisk. With Sans, Monster Kid tends to be more silent and submissive. When only around Frisk, Monster Kid is very loud and prone to name-calling.
  • They will tend to hold their head against Frisk's back to make sure they do not get separated.
  • Monster Kid was blinded by unknown bullies who thought it was funny to make sure the young monster didn't see again.
  • They are half foot shorter than Frisk.


  • Napstablook used to live in a house within Waterfall with their cousins, being the one that would lounge around the small home.
  • They were once "happy" being around their cousins, even if the affection wasn't often shown.
  • Napstablook was left behind when their cousins wanted to find their own ways to live. Napstablook was angry at this act, but they knew they couldn't always be around the other two in the UNDERGROUND. The cousins at least visited the ghost and brought things they thought the ghost would like.
  • Both cousins turned up missing one day. No more visits, no more talks, causing the ghost to become more passive-aggressive than they normally were.


  • Former leader of the royal guard and Asgore's personal bodyguard.
  • She was once a girl with a bright and wishful personality, being friends with the skeleton brothers and the ghosts. That changed when Asgore got his hands on her and brainwashed her. He changed her into his own little toy, and she took orders diligently.
  • Whenever Asgore didn't want to get his hands dirty, Undyne would often be the one to do what she had to to please their king. Because of this, her body was almost completely engulfed in fire.


  • Alphys was once a very down-to-earth person, but after being in the UNDERGROUND for so long her common sense started to fail her.
  • The only thing that was keeping her just a bit sane was Undyne. She believed Undyne was the only one who enjoyed her company and actually liked talking with her.
  • Over time, the two became lovers. It worked out nicely even after Alphys was appointed Royal Scientist and Undyne was appointed Head of the Royal guard and Asgore's personal bodyguard.
  • When Sans killed Undyne, Alphys' mind finally snapped. She killed many monsters that helped her around the lab and some that just wanted to "visit their family".
  • Just like Sans, she despises him for what he did. She thinks Papyrus got what came to him for not killing the "traitor".


  • King of the UNDERGROUND.
  • He loathes humans and will kill any that he sees.
  • He believes that he did the right thing with his son, though in the back of his mind he wonders if he could have done something differently.
  • Asgore still thinks that his wife is still alive, but just in hiding.
  • He wants seven SOULs so that he gains enough power to become a god to kill all of the humans in the world. He only has five.


  • Once was Sans' best friend.
  • Muffet was one of the first monsters to be killed when the others started to kill each other.
  • She was slain protecting her spiders.
  • Her spiders can still be seen in Hotland, but they are very aggressive and hateful to monsters other than Sans.
  • The spiders do seem to take a slight liking to Frisk.


  • One of Napstablook's cousins.
  • Mettaton was the more kind and yet ambitious of the three cousins.
  • When Alphys told them of a chance to help others and be a star, Mettaton couldn't help but accept. Unfortunately, it wasn't the fame they wanted.
  • They were given a robotic body, but he was subjected to torture and the like while being watched by other monsters.
  • Alphys didn't want to give them fame, she just wanted to see how much shock it would take to break a monster.
  • Just like Mad Dummy, he "disappeared" without a trace.

Mad Dummy

  • One of Napstablook's cousins.
  • Mad Dummy sought to be helpful to others, though not in the way he wanted.
  • He was beaten by every monster as a way to train themselves. They seemed to like it more because he would scream with every hit.
  • When he wasn't being a training dummy, he would go and visit his other cousins.
  • Undyne was the last person to be with him before he disappeared.


  • A sad bunch that Alphys ran tests on.
  • Not much is known, other than that Alphys forced DETERMINATION into the monsters and created them.
  • She killed some of them off, leaving only Endogeny and Memoryhead.
  • Endogeny is the only docile and affectionate of the last two amalgamates, whereas Memoryhead is more manipulative and relentless.

Location of Fire Meanings


  • A head with fire on it shows that the individuals suffered from psychosis, killing someone due to "not being themselves".


  • Eyes are common with people that killed out of vengeance, no matter if the person that they have avenged is dead or alive.


  • This location is for those that killed others with lower HP.


  • This is a less common area to have fire, but there are some with it in this location. This place tells those that see it that this individual mercy-killed, killing others out of love or necessity.


  • Any fire on the chest plague those that killed a relative or friend, and immediately regretted that action.


  • As disgusting as it is, the stomach represents survival by eating others. This is not very rare in populated places.


  • The back will indicate regret for killing a stranger. Monsters or humans with these are often kind, or at least feel terrible about hurting others.


  • Boredom causes many monsters to do rash means to have fun. Fire on the arms are usually seen as trophies since others gain them by killing monsters with high HP.


  • Hands reveal those who back-stab. Those with this location gained the trust of another, only to take away their life. This seems common among former friends or monsters with humans.


  • Survival by killing someone to protect another. This is one of the rarest locations in the underground, due to most monsters seeming selfish.


  • This indicates an unintentional kill. People with fire on their feet are usually innocent bystanders, weak or kind individuals, or those that accidentally kill others.

Route Instances

In the AU, there are four routes available, plus a special instance called "Ashen!Frisk".

Neutral Pacifist

  • A route where Frisk will gain trust and love from Sans and will always try to stop him from killing/hurting others. Happens when most of the monsters are spared, from both Frisk and Sans.

True Neutral

  • A route where Frisk cannot always influence Sans' actions. Occurs when Frisk hasn't received enough trust from Sans and when almost half of the monsters are killed by Sans or Frisk.

Genocide (Frisk)

  • A route where Frisk kills everyone, but it is not known if they make it off of the mountain.

Genocide (Sans)

  • A route where Sans kills Asgore and Frisk. He takes their SOULs and the other humans’ SOULs in hopes he can reset to the point before Papyrus is killed, by killing Undyne before that.


An instance where Frisk had to take Sans’ life as a way to stop him from suffering from the fires that plagued him. Permanent ash covers their body and a small fire shines on their chest, reminding them of the friend they once had to mercy kill.

  • This version of Frisk is very melancholy and is usually alone, except for Muffet’s spiders. There are some instances where Shyren will be with Frisk, but that's rare. They don’t like getting close to others, both physically and figuratively; this is more a physical fear since the fire on their body is known to set things ablaze if they weren’t paying attention or got too angry.
  • There are also some cases where they won’t “be themselves”; they are harboring four SOULs, including theirs due to them unintentionally absorbing the other SOULs. The SOULs are of Chara, Asriel, and Sans. Whenever Frisk is frightened, stressed, or emotionally drained, one of these three will take over the human’s body. This, though, often leads to them arguing over who will use Frisk and causes the human to get a massive migraine.
  • Though they are not in direct control of Frisk's body, the voices and the emotions of the three SOULs can influence their mood and actions to a degree. Whenever they grow silent, it's because they are listening to whichever voice in their head is talking.
  • Frisk’s appearance also seems to change when someone takes them over, but only slightly. If Sans takes over, one eye changes to look like a ring of fire while the other glows brightly, as well as what looks like bits of red smoke coming out of their mouth. If Asriel takes over, both eyes will look like swirling rainbows and they will have dark markings on their face. If Chara takes them over, their eyes turn a darker shade of red (almost black) and a streak of hair will be a more reddish-brown color.
  • Should the three cooperate and have the four resonating together, Frisk, Sans, Chara, and Asriel can all share the human's body at the same time.
  • In technicality, Ashen!Frisk would be considered LV 18 (one LV short of Sans) instead of 19. This again is due to another mercy-kill.

Shared Abilities

Flames of Karma

Sans’ shared ability. Uses the fire on their chest to cloak the kitchen knife in flames. Despite Sans wanting to kill those that threaten Frisk (even if he doesn’t have a physical body) and with it being a means of offense, Frisk will refuse to let Sans do something rash. They will only use this to weaken enemies to get away from danger.

Prism Protection

Asriel’s shared ability. Plunges the knife into any surface, whether it be a wall or the floor. Pillars of colors appear in front of them. The pillars act as shields for a short while, allowing them to find a way from either hostile monsters or humans.


Chara’s shared ability. Through sheer DETERMINATION, HP increases tremendously. If HP is low, this ability will instead give them a few extra HP to “power through” any pain.