Souls

One of the largest questions on the minds of those with the time for it is that of the Outsiders. Why do they appear? What is their purpose? If one of these thinkers were to have a short conversation with a midwife, they'd probably discover a telling clue: it is believed by many midwives that the vast amount of children born here possess no souls as they do not cry when born. These same children will never show any extreme emotion, whether happiness or sorrow, and often are noted by those with souls as bland and seemingly grey-scale.
Souls in the Demiplane of Dread are in constant shortage. The vast amount of people needed to entertain the Darklords rarely need any sort of depth to them, thankfully, so the souls are given to those The Mists determines will need them. To increase the number, Outsiders are brought in, used to enhance a darklord's torture, and then not allowed to leave so as to ensure their souls enter the cycle.
The people most likely to possess a soul are those the Mists intend to hold importance later in life, such as adventurers and forces of good or servants to a Darklord. The most famous soul is that of Tatyana Federovna and her reincarnations. Like all other examples, Tatyana's particular reincarnation always appears the same and recalls none of the previous lives except in echos, such as a strong feeling of knowing a place she's only seen in a past lifetime. Beyond that, the exact personality of the reincarnation has been seen to vary largely from situation to situation, with only the strong echo of revulsion towards Strahd von Zarovich managing to stay constant. In this it can be gathered that only the most potent tendencies stay constant from life to life for a single soul, allowing for some measure of freedom to push back against fate.

Manifestation

The usual way for most to determine if someone has a soul or not is to see if they'll cry (or express some other emotion), though this isn't reliable. With this method traumatized child and a soulless one are indistinguishable, as it's impossible to tell why they didn't to laugh or cry. Fuged with the rare ability to see souls, such as Klaus von Zarovich, are the only ones who can tell with complete surety.

Localization

The phenomenon of the missing souls is widespread, but is most common in the newly formed domains that were not taken directly from the Prime Material Plane.

Type

Metaphysical, Divine