From Glitchtale Wiki
A Monster soul is a unique and vital component for Monsters. This type of soul does not possess the ability to have traits.
A Monster soul's composition is theorized by Monster-kind to be made up of love, hope, and compassion. However, the true composition and nature of the soul is unknown. Monster souls are rather weak, shattering when the monster's body turns to dust. It's said that the reason for this is the lack of Determination, the substance that allows a Traited soul to persist after death.
Boss Monster Souls
Boss Monster souls are different. They possess the same composition as a normal one. However, they're stronger and more resistant, which allows them to persist for some seconds after their body turns to dust. A quality exclusive to Boss Monsters is that when they have children, a link is established between them. Deriving from this, the parent’s full magical production becomes split between them and their children. The divided power to the latter is a constant stream of magic, with the purpose being to help them grow stronger. When the child reaches their "prime", the magic of the link returns to the parent.
- If a Boss Monster’s child were to die, the magic of the link would return to the parent.
- The Boss Monster link can be forcefully reversed, like how Gaster did it. However, it is extremely taxing. It causes the parent's soul to crack due to the magical overload (the magic from the link returning to the parent all at once) and can't happen through normal means.
- Monsters cannot handle HATE inside of their souls or generate it.
- Monsters can generate small amounts of Determination due to their newly obtained physical matter throughout the events of Season 2. This can be seen with Mettaton and Sans.
- Among Monsters, there's a rare kind of magic called "Chroma Magic", which allows the Monster to mimic each one of the seven Human soul traits. An example of this is Gaster's Chroma hands.