Ghosts

The only thing known to destroy a soul is the Soul Destroyer which leaves behind a ghost, an empty shell where once a Darwinian stood.

Details
Ghosts move as normal Dariwinians would be, but can no longer react to the world anymore and ignore everything around them, their bodies slowly rise up and disappear. They can remain for a number of minutes as they slowly fade away. The ghosts are said to linger where they died even after fading away and are unable to move on to any form of afterlife. To the Darwinians, this is the closet they have to true death.

Such lost souls are never reborn and are removed from the cycle of death and rebirth. With it is lost all the Darwinian digital DNA, the data containing the past lives and experiences of each Darwinian that held the soul is lost forever. This is both a great loss to the Darwinians, but also to the research of Dr. Sepulveda himself.

Multiwinia
In the lore of Multiwinia, remaining versions of the Virus that lost interest is taking the normal Darwinians and instead began to gather up the remains of the lost souls left behind from the Soul Destroyers. This in turn caused the viruses to turn a ghostly grey colour.

When Viruses are destroyed, they produce a "soul" that can be harvested, coloured in a grey colour. Though this is the soul of the virus, which is born from the souls of previously consumed Darwinian from the time they fed on Darwinian souls. It sadly is not one of the lost souls, meaning that even now, these lost souls do not know peace.

Trivia

 * Ghosts are traditionally believed to be the spirits of people unable to move onto the afterlife for whatever reason. This can be either as punishment or because they have unfinished business, though the reason for such hauntings is debatable per culture around the world.