Monster

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

Overview
King Asgore Dreemurr ruled all monsters in the Underground. After The War of Humans and Monsters, monsters moved to the Underground from the Surface, which they ruled over with the humans until war was declared on them. It was presumed that at least three-hundred-and-twenty-two monsters inhabited the Underground—given the number of monster souls visibly emanating from Asriel Dreemurr as the barrier was broken, plus incorporeal ghosts, who did not participate in the event. At least one-hundred-and-four monsters were killed during each of Frisk and Chara's Genocide Routes. Despite the negative implication of the word "monster," monsters were not an evil species. It was even said that monster SOULs were made of love, hope, and compassion.

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

Physiology
Monsters came in numerous shapes and sizes, although the notable species of monsters included skeletons like Sans and Papyrus, ghosts like Napstablook and their relatives, and Boss Monsters like Toriel and Asgore, the latter of which very likely being the only kind of monster that could live eternally. Using Asgore as an example, he fought in the war between humans and monsters and still lived many years later, existing when the first human fell and when the Frisk fell as well. An explanation for this was provided by Gerson when Frisk talked to him after the barrier broke. Monsters SOULs were usually wholly white, with a couple of exceptions.

A monster's body was mostly made of magic and was attuned to its SOUL. When a monster got old, they entered a state called "fallen down." A monster in this state laid immobile and turned to dust shortly afterward. When a monster died, its body turned into dust. This dust was often spread onto the owner's favorite thing in funerals so that their essence lived on in their possessions. Monsters' bodies and SOULs were closely linked; when a monster died, their SOUL was lost forever. Boss Monsters were an exception, as their SOULs existed outside their bodies for a few moments before shattering.

Because of their magical nature, monsters' bodies could not handle high concentrations of determination. When monsters were injected with it, their bodies liquefied and melted together. The Amalgamates were a clear example of this.