Curse of Strahd
A myserious, spectral rider on a skeletal mount.
In the Campaign:
Chapter 2: Death House
As Elizabeth and her company approached Borovia Village the night of their arrival, the skeletal rider rode forth from the mist. It locked eyes with Elizabeth and dropped a black book before riding into the mist. Calos tried to follow it to no avail.
Chapter 7: Mad House of Vallaki
The Skeletal Rider returned in a cloud of fog as the party neared Krazk, where he dropped a hand mirror with a medusa sculpted on the back before vanishing again.
Nothing is known about this strange phenomenon, whether it is friendly or not, its intentions behind dropping the book or even where it came from in the first place. It seems like everybody in the group has their own theory about it.