Souls are something that aren't really known about aside from some theories. For the most part, people use the term "soul" as something that is.. there? But isn't really!
However, souls do exist and are way more important than most would think.
A normal human soul is a white heart, it's about the size of the person's hand. It cannot be seen in most circumstances, which is why most would believe they don't have an actual soul!
Souls can only be seen when interacting with soul magic. Soul magic consists of using written down spells, either saying them out loud or in your head! The spells can do a bunch of different things depending on the type! Souls can be seen as the person is performing the magic! There are also spells that can view someone's soul as well!
Magic is no longer used in the modern (2015) world. And when it was used, it was seen as an evil thing. During the 16 - 17 hundreds, people would call witches evil and deserving of death. There is a... bit of hypocrisy in this though. The same people claiming soul magic to be evil, used it themself for holy spells! ^^
Note: only humans / creatures with a soul can use soul magic.
When a person dies, their soul shatters. These shards are known as Fragments. These Fragments are jumbled up memories and emotions of the person that had that soul. These fragments cannot be seen or heard by anyone. They are non physical and just there to watch.
The only exception is if the fragment was murdered. That will cause them to be bound to the perpetrator's soul. The one responsible will be able to see and hear the Fragment. Essentially this is the Fragment's version of haunting someone!
Fragments will fade in and out of "existence." What does this mean? Basically they will appear and then disappear. Instead appearing into a pitch black area that is essentially just being asleep forever. That is "outside of existence." They are unstable and cannot control when they appear and how long they stay for before they vanish again.
Multiple fragments of the same soul cannot appear at the same time. Along with that, fragments from different souls cannot interact with each other (see/hear/know they exist). Fragments can act different each time they appear, as they've become a jumbled up mess of emotions and memories.
β Parasites cannot see fragments
β If a parasite kills someone in someone else's body, the fragment will attach to the human's soul
While most souls are pure... cursed souls appear discolored and grey. These souls are cursed at birth to be victims to parasites. This curse is some mutation caused in the soul that rarely appears on its own. However, it can be passed down into the next generation if the human as a cursed soul! It doesn't need to go through every single generation either. Instead it can appear back in later generations if the parent doesn't have it!
As the soul gets more and more drained, it'll become darker and darker. Until it is pure black. Once a soul is pure black, the human possessing it dies. The soul stays within the body, unlike fragments.