Monster

For a list of all known monsters, see Category:NPCs or Category:Characters.

Monsters (JP: モンスター) are a very diverse race in Undertale. They dwell in the underground beneath Mount Ebott, while humans live on the surface beyond the barrier.

Besides the protagonist, the fallen human and Flowey, most characters encountered in the game are monsters. The game mentions a few real-world animals, such as mice, spiders, and dogs. It is unknown whether those count as monsters.

Overview

King Asgore rules all monsters in the underground. After The War of Humans and Monsters, the humans from the surface trapped the monsters underground. Despite the negative implication of the word "monsters", monsters in Undertale are not an evil species. It is even said that monster SOULs are made of love, hope, and compassion.

Monsters are far weaker than humans, both in physical combat and regarding the strength of their SOUL. This inadequacy is because monsters are mostly made of magic, which in turn makes them attuned to their SOUL. The damage a monster receives from an attacker is dependent on both the monster's will to fight and the attacker's will to hurt. Monsters, however, can obtain an "unfathomable" amount of power when they absorb the SOUL of a human.

Physiology

The SOUL of a monster

Monsters come in numerous shapes and sizes. Most monsters physically appear as anthropomorphic, non-human beings (animals, natural/artificial objects, supernatural beings, etc.), with the most notable species of monsters including skeletons like Sans and Papyrus, ghosts like Napstablook and Mad Dummy, and Boss Monsters like Toriel and Asgore. Monsters SOULs are upside down and wholly white.

A monster's body is mostly made of magic and is attuned to its SOUL. When a monster gets old, they enter a state referred to as having "fallen down". A monster in this state lies immobile and dies shortly afterward. When a monster dies, their body turns into dust. This dust is often spread onto the owner's favorite thing in funerals so that their essence lives on in their possessions. Monsters' bodies and SOULs are closely linked; when a monster dies, their SOUL is lost forever. Boss Monsters are an exception, as their SOULs exist outside their bodies for a few moments before shattering.

Because of their magical nature, monsters' bodies cannot handle high concentrations of determination. When a monster has too much determination, their bodies liquefy and melt upon death. The Amalgamates are a clear example of this, as well as Undyne when killed.

Trivia

• Monsters turning into dust when they die possibly references to certain battle texts that explain what happened to an enemy when they are defeated in Mother 1 / EarthBound Beginnings and MOTHER 2 / EarthBound. When an undead enemy is defeated, the battle text explains that the enemy "turned to dust" or "returned to the dust of the earth".
• The term "fallen down" possibly originated from the Japanese-only Mother 2 Official Guide Book, which contained an English headline reading "Falling Down of Our SweetBogy Tent", describing the defeat of the monster Boogey Tent. In this case, "falling down" was a mistranslation of "倒される", Japanese for "defeated", which literally means "knocked down".
• It is presumed that at least 322 monsters inhabit the underground — given the number of monster souls visibly emanating from Asriel as the barrier is broken, plus Napstablook, who did not participate in the event. New Home, which is largely unexplored in the game, is said to be a crowded city, suggesting a large off-screen population. At least 104 monsters must be killed in order to complete the Genocide Route.
  • If the Ratings during the Mettaton EX battle are taken as the number of monsters watching the battle, then there are well over 10,000 monsters inhabiting the underground.
• Boss Monsters are very likely the only kind of monster that can live eternally. An explanation for this can be found in one of Gerson's dialogues during the epilogue: Boss Monsters stop aging if their offspring dies.