The devil Strahd is a curse placed on the land because of a forgotten sin of the Barovians' ancestors. (This is untrue, but the Barovians believe it nonetheless)
A vampire must rest in its coffin during the day
At night, it can summon wolves and vermin to do its bidding
A vampire can transform into a bat, a wolf, or a cloud of mist
In its humanoid form, it can dominate you with its powerful gaze
A vampire can't enter a residence without an invitation from one of the occupants
Running water burns a vampire like acid, and sunlight causes a vampire to burst into flames
The Land of Barovia
Anyone who attempts to leave the land of Barovia begins to choke on the fog
Those who don't turn back perish
Many strangers have been drawn to Barovia over the years, but they all die or disappear before long
Wolves, dire wolves, and werewolves prowls the Svalich Woods (C), and hungry bats fill the skies at night
The fortified village of Village of Krezk (S) lies at the west end of the valley and is built around an old abbey
The village burgomaster is named Dmitri Krezkov
Van Richten wrote a series of books, detailing all he knew about monsters of all sorts. The 'Van Richten's Guide to the [monster]' series
Perhaps your players find a copy of one...perhaps when they succeed on a monster-knowledge check, you tell them that they remember reading [fact] in the book Van Richten's Guide to [monster].
If the players do something impressive involving slaying monsters, have NPCs compare them to the legends they have heard of Van Richten.