Boss Monster

For the list of bosses fought in Undertale, see Boss.

A Boss Monster (JP: ボスモンスター) is a special species of monster who distinctly has SOUL persistence. Known Boss Monsters rule over the underground. Although all Boss Monsters are also bosses in Undertale, not all bosses are Boss Monsters. All Boss Monster encounters activate Serious Mode, and all of them appear as a final boss in at least one of the game's endings (including Hard Mode).

Overview

The game's lore explicitly states two things:

1. Boss Monsters' SOULs persist briefly after death.
2. Mature Boss Monsters only age when their offspring does. This explains Asgore and Toriel's longevity as their son, Asriel, died at one point, and they therefore cannot biologically age.

When a Boss Monster dies, their body turns to dust; however, their SOUL, in an upside down heart shape, lingers for a few seconds before shattering. Similar to other monsters' SOULs, the SOULs of Boss Monsters are white.

There are three known Boss Monsters, all of a royal bloodline: Toriel, Asgore, and their son, Asriel. Although not stated to be intrinsic characteristics of Boss Monsters, the three of them share several traits. Physically, their heads resemble Nubian goats, and have a pair of horns, long droopy ears, and visible fangs. All three have anthropomorphic bodies and paws at the end of their limbs. Toby Fox also stated that Toriel's appearance was partially based on the Mimiga, a fictional species from Cave Story. The Dreemurrs also share an affinity for fire magic, and wear the Delta Rune emblem.