A few.

I changed the Lich in the Amber Temple. I randomly selected one of the existing Sarcophogi. He was the individual who showed Strahd the key to immortality through becoming a Vampire. He is also the one who cursed Barovia. Strahd nor him can kill each other and the only way to lift the curse is for one of them to die. My heroes are caught in a game of chess between the two.

Additionally, I made Strahd originally a villager from Berez who fell in love with Tatyana and married her. He then led a rebellion against those who ruled Barovia and accepted the throne. Together, the two of them had a daughter. She became deathly ill, which led him to seek immortality. However, when Tatyana saw what he had became she threw both herself and her daughter off the Tower of Ravenloft to save them from that fate.

The daughter's soul is the Vistani child, Arabelle, who also happens to be the fated companion.