After another cutscene, you'll find the duchess' palace attacked by vampires. Geralt will have to kill another bruxa. This one is a easier than the first, as she doesn't go invisible as often. Use yrden to trap her and attack during openings.
After a conversation about what to do, follow Regis to the next step of the mission. Several vampires will intercept you along the way, but Regis will be of assistance this time. You're looking for Damien. Eventually, you find tracks and a scent. Follow that scent onward while using witcher sense. Watch out for garkains and fleders along the way. Vampire oil is the best way to deal with them.
Upon reaching Damien, he'll tell you the location of Syanna. Head to the playroom after the conversation with him. Walk upstairs and use witcher sense to find a the diary that outlines the duchess' and Syanna's childhood. After reading each passage, look for the book The Land of a Thousand Fables. It's in the locked cupboard. The key to it can be found behind the picture frame above the staircase. Complete the quest by opening the book.