Ludmilla is the eldest of Strahd Von Zarovich's brides. Around 200 years ago, she went to Castle Ravenloft to ask Strahd to teach her magic. He agreed and she became his apprentice, eventually becoming a powerful wizard with a specialization in divination magic. She also sought immortality, which she obtained in becoming a vampire.
At some point, before marrying Strahd, Ludmilla had one or more children with someone else. As such, she has several descendants who are not related to Strahd, including her great-great-granddaughter, a seven-year-old named Arabelle.