Count Strahd von Zarovich
Count Strahd von Zarovich is a vampire and the ruler of Barovia. The waffle crew eventually came to Castle Ravenloft and found Strahd, who was weeping in the crypt of his parents. Evelyn stunned him with the holy symbol of ravenkind and Strix trapped him in the Strahd doll.
When the group went into Barovia's past, they battled Strahd while trying to save Sergei and Tatyana's lives. Diath stabbed him in the back with Gutter, wounding him. Strahd was able to recapture Tatyana and Sergei, and he became the dark lord of Barovia.
When Strahd was a mortal, his younger brother Sergei was set to marry a woman named Tatyana. Strahd was in love with her, but she did not feel the same about him - she thought he was too old. On their wedding day, Strahd killed his brother and tried to profess his love to Tatyana. When she saw what he had done, she jumped off a balcony to her death.
"Death", or the dark powers of Ravenloft, took notice of Strahd's evil act. Strahd turned into a vampire and went on a rampage, killing his guards and the wedding party. His realm of Barovia became shrouded with mist and Strahd's curse took hold. Over the many years, women would be born in his domain that looked and acted like Tatyana. Whenever he discovers this "reincarnation", he tries to make her his bride, but he always fails.
Strahd and Paultin
Strahd tried to turn Paultin against the waffle crew, but was unsuccessful. Ultimately, the group trapped Strahd inside a doll. The doll eventually ended up in the hands of some shadar kai from the Shadowfell.
Diath found the outer skin of the Strahd Doll in Waterdeep.  Soon after, the group encountered Strahd at a party held by Lady Margaster. The heroes attacked and killed him. When slain, he assumed gaseous form. Evelyn cast moonbeam on him, which reduced him to a pile of ash. Strix collected and kept some of Strahd's ashes.
Strahd has appeared in many D&D products in many older editions. He has a appeared in Ravenloft, the Ravenloft Campaign Setting boxed set, Return to Castle Ravenloft, Open Grave, and more.