Monster

Monsters are a very diverse race that dwells in the Underground beneath Mount Ebott, while humans and other races/species live on the surface beyond the Barrier.

Overview
King Asgore Dreemurr rules all monsters in the Underground. It is presumed that at least three-hundred-and-twenty-two 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 one-hundred-and-four monsters must be killed as part of fulfilling the requirements of the Genocide Route. After the War of Humans and Monsters, monsters moved to the Underground from the Surface. Despite the negative implication of the word "monsters", these monsters are not an evil species. It is even said that monster SOULs are made of love, hope, and compassion.

Monsters are by 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 tun 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 and "unfathomable" amount of power when they absorb the SOUL of a human, which was originally what began the war between the two species.

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. 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 Frisk fell as well. An explanation for this can be found if Frisk speaks to Gerson in the epilogue of a True Pacifist Route. Monsters' SOULs are usually a wholly white, with a couple of exceptions.

A monster's body is mostly made of magic an is attuned to its SOUL. When a monster gets old, they enter a state called "fallen down". A monster in this state lies immobile and turns to dust shortly afterward. When a monster dies, its body turns to 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.