The Vistani

The Vistani (singular: Vistana) are wanderers, traveling about in horse-drawn, barrel-top wagons, which they build themselves. Compared to Barovians, they are flamboyant, dressing in bright clothes and laughing often. As much as they feel at home in Strahd's dreary land, they know they can leave it whenever they please and aren't damned to spend eternity there.

The Vastani and Strahd

Davian claims that long ago the Vistani saved Strahds life during a time when he was mortal. Strahd allows the Vistani to come and go through the fog to repay his dept.
The Vistani act as agents of Strahd. However, in the centuries since Strahd became a vampire, many Vistani have allowed themselves to be corrupted by Strahd, to the extent that they consider him their king. Vistani who serve Strahd are less lively and friendly than normal Vistani, and their hearts are poisoned with dark intentions.