Ghosts

Ghosts are a form of undead — usually but not always Medium in size — well-known across Barovia and most other realms.
Ghosts can be born for many different reasons, but the most common unifying factor among them is unfinished business. Unlike some other forms of spectral undead such as specters, shadows, or wraiths, ghosts are not inherently evil. For instance, the first ghosts our heroes encountered in Barovia were those of Rosavalda and Thornbuldt Durst, a pair of children who had died in their ancestral home many years before. After setting the children's remains to rest, their spirits seemingly followed, no longer bound to the place where they'd died.
However, in the event a malicious ghost is encountered, it can have dire consequences, even for seasoned adventurers; in their fight on the Ethereal Plane in the aftermath of the Battle of the First Two, the party had many of their companions age several decades as a result of hostile ghosts' abilities.
In Barovia, recently diseased souls linger as a sort of proto-ghost that may be unique to this realm and its hold on the souls of all who die here; unable to travel to the Fugue Plane or any other such place, the spirits of the newly dead linger in a form that is similar to that of a ghost, though not in possession of a ghost's full powers. Almost the entirety of the party has spent at least some time in this form, and have discovered its advantages, with Sylvain, Osrin, and Bodaway all having used their incorporeal forms to do reconnaissance for the party.