Boss Monster

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

Boss Monster
A Boss Monster as seen in the Undertale introduction.
Mechanic type	Plot Mechanic

A Boss Monster 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.

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 necessarily 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. The Dreemurrs also share an affinity for fire magic, and wear the Delta Rune emblem.

Trivia

• The goat theme among known boss monsters may very likely be a deconstruction/reconstruction of typical devilish traits, as devils often have goat traits and goats are used prominently in Satanic symbolism. However, this is made mundane since all known boss monsters are essentially anthropomorphic goats. Many powerful RPG bosses are devilish, hence the name and significance also back this up.
  • The fact that they all can use fire magic and Asgore using a trident also supports this, as the devil is often associated with fire and the trident.