Souls

"What comes after death? Does it matter, if the life that preceded it was a life well spent?"

— unknown scholar

Mortals in the Brass realms are being of spirits and body. They are bound to material things, material pleasures and material woes, unlike Spirits.
While the body of amortal can be killed, their soul can not. A soul can be damaged, ripped apart, traumatized and devoured but never truly destroyed or killed.

The power of souls

Whilst alive, the power of a mortal soul is greatly limited and does not grow. Nevertheless, the soul can influence the world to an extent and shape it according to its beliefs. It is for this reason that rituals and customs to appease evil spirits are actually effective even if they are nonsensical and why it is possible to summon Spirits without being able to cast Magic.

Chains of Words

Promises and oaths made on one's name, power or soul have real true weight in the brass realms. For swearing an oath on these things puts them at tanguble risk whenever such a promise is broken.
Breaking too many such promises will lead to a person's very soul betraying them at crucial moments, such as a mage's powers failing them in battle or a warriors swing to tremble and miss. Breaking too many vows can even lead to mental illness and insanity, at the extremes.

This does not mean that other promises can't be broken, they can. It is simply a very bad idea to make a vow and putting one's soul at risk if one does not truly mean to make sure to keep that promise. For this reason such promises are often invoked only where honor can be guaranteed. Promises made on one's power and soul are a crucial part of The Old Code.

Death

In the Brass Realms, if a mortal dies, their soul is harvested by a Reaper, crosses the veil and moves on into the Spirit World. From the moment of their death they become a Shade
From there what happens now to the fledgling spirit is heavily influenced by what the mortal did throughout their life.

The Dead

If the soul was very loyal to an ideal or concept, either through actions or beliefs, it might be drawn to one of the higher realms, such as Hades if they were particularly selfish or Mount Celestia if they lived their life for others.
If they person was an avid worshiper of a deity, they might be taken into the realm of that being and join its followers there.

The soul will then inevitable join the inhabitants of that spiritual realm and become one of them. They might become a demon or angel, striving to advocate the cause of evil or good respectively, learning the secrets of the universe over centuries and milennia.

This entire process will be perceived by the deceased in whatever way makes most sense to them. Accounts by those that have returned from near-death experiences can thus vary greatly.

The Lost & The Damned

The Lost: A grim fate waits for those who are unable or unwilling to move on to an afterlife. A lost soul will travel from the border of the spirit world into the spirit world proper and go from spirit domain to spirit domain, looking for a place to stay. If no deity invites them to their domain, inevitably they will be caught by one of the many predators that roam the spirit world and devoured.

The Damned: The arguably worst fate a soul can meet is those unfortunate few who are unable or unwilling to leave the border of the spirit world entirely.
These unfortunate few will stay shades forever. They will drift for centuries, hoping fruitlessly to come back to life or obsessing over the mortal world. In the end these souls slowly lose their memories and personality until they finally dissolve into the background energies of the spirit world.
In this way they eventually do contribute to the great working, even if they no longer witness it.