These are the souls of the deceased who enter Helheim, they appear primarily as black featureless human silhouettes and appear to be entirely indifferent to anybody or anything as they did not react at all when Kratos or Atreus went near them or even through them. Though they appear to be mute, if one listens very closely you can almost make out quiet whispers over the howling winds.
They can be seen several times while in Helheim making their way to the Bridge of the Damned to get to the City of Helheim. Because the Valkyries have been prevented from doing their job, Helheim has begun to overflow with too many souls at a time and so the gates leading to the bridge has been locked trapping an ever increasing crowd of souls outside and preventing them from reaching the city.
Certain souls are either unable or unwilling to rest peacefully and so become Hel-Walkers to escape Helheim and attack the physical world.