Monsters


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

Monsters are a very diverse race in Undertale. They dwell in the Underground beneath Mount Ebott, while humans and other races/species 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 Dreemurr 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
Monsters come in numerous shapes and sizes, although the notable species of monsters include skeletons like Sans and Papyrus, ghosts like Napstablook and their relatives 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, its 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 monsters are injected with it, their bodies liquefy and melt together. The Amalgamates are a clear example of this.

Trivia

 * It is presumed that at least 322 monsters inhabit the Underground – given the number of monster souls visibly emanating from Asriel Dreemurr as the barrier is broken, plus incorporeal ghosts, who do not participate in the event. At least 104 monsters must be killed as part of fulfilling the requirements of the Genocide Route.
 * Boss Monsters are very likely the only kind of monster that can live eternally. Using Asgore as an example, he fought in the war between humans and monsters and still lives many years later, existing when the first human fell and when the protagonist fell as well. An explanation for this can be found in one of Gerson's dialogues during the epilogue.