Madam Ava, leader of the Vistani, knew Strahd before a curse befell him. Strahd has become their commander. Madam Ava is bound to not talk of the curse. The Vistani are special, they do his bidding in return for safety.
Mordenkainen
Madam Ava gave us details about a mighty wizard who tried rallying the people of Barovia against Strahd: Mordenkainen Story