It's time to attend the ball. Put suitable clothes on Geralt to attend the Mandragora in Hauteville. Head to Tailor's Workshop near Coopers' Gate and buy the elegant beauclair doublet before traveling to the soiree.
Once inside, you can partake in the party's festivities or look for the Koviri Orchid that belongs to the woman who can give you more information about The Cintrian. She's in a private room along the left side of the party. Once you find the woman with the orchid, you'll learn she's not the person you're looking for. From there, use your witcher sense near the refreshment table to find the heart-shaped box. You'll catch Cecilia Bellante's scent. Follow it using your witcher sense. After a conversation with the guard, head upstairs to find the singer. Cecilia has been killed.
Duchess Anna breaks off at this point. Head to the room's balcony and track evidence using your witcher sense. Finding all the evidence will trigger the next cutscene. After a lengthy cutscene that lays out the next step in Geralt and the duchess' plan, the quest is complete.