Cut off point: this quest is a cutoff point for the secondary quest The Warble of a Smitten Knight. If you haven't completed that quest yet, it'll fail and therefore you won't have access to the other quests A Portrait of the Witcher as an Old Man and Of Sheers and a Witcher I Sing.
Important: once you begin this quest, you won't be able to go off and do other quests until the main storyline is finished, so make sure you're prepared for that.
Meet Damien at Count de la Croix's Mill at midnight and let him know when you're ready. Note: between this and until you get to the first part of the next quest, you won't be able to do a manual save, so make sure you have a recent one.
After the cutscene, take out all the guards here, then make your way further into the courtyard, taking out the guards as necessary. Eventually you'll find Roderick with more backup. Cut them all down and talk to Roderick to learn something isn't right in regards to the hostage and Syanna. You can help him or not afterwards, then carry on, taking out guards as you go. There Can Be Only One info: helping Roderick here counts as part of the chivalric value "compassion".
After you take out a group of archers on a narrow balcony, head up the stairs for a cutscene and where you're now joined by Dettlaff and Regis. As long as you move at a slow enough pace, they'll do all the heavy lifting, taking out almost everyone that gets in your way so you may only get a few swipes in.
Approach the manor and another cutscene starts where you see the truth for yourself: Rhena and Syanna are one and the same person. Answer however you wish and afterwards Dettlaff will order her to meet in 3 days to explain herself or else he'll attack Beauclair, before flying off. Despite Syanna stating she'd meet him though, on exiting the building Anna Henrietta, grateful to see her sister, orders her held in a safe location and demands Geralt to find and kill Dettlaff before he makes good on his word and attacks the city. This will then end the quest and immediately jump into the next quest, 3 days later.
- Ducal guardsmen commanded by Captain Damien de la Tour were to aid Geralt in storming Dun Tynne - this was the duchess' wish and command. The briefing before the assault would be held at the mill where Dettlaff had recently dispatched Count de la Croix in cold blood.
- Within Dun Tynne's walls, the witcher came upon Roderick. The old knight had erred many times in his life, yet never as gravely as he had when opening his castle to a group of bandits, a vandaguild. He had been persuaded to do so by none other than Syanna, Duchess Anna Henrietta's sister. The young woman had clearly captured the lord of Dun Tynne's affections, or perhaps merely aroused his passions. This achieved, she had quickly wrapped him round her finger. Interestingly, Roderick seemed to know Syanna well, yet he had heard nothing of her abduction. Having questioned the wounded knight as much as he wished, Geralt set off for the castle's keep, in a room of which he expected to find Anna Henrietta's dear sibling.
- While storming Dun Tynne, Geralt was surprised to find he had two allies - Regis and Dettlaff. Naturally, the witcher assisted by two higher vampires plowed through the castle's defenders about like the Nilfgaardian conflagration had plowed through Cintra. At long last they reached the room where Geralt expected to find Syanna... And he was not disappointed. Yet it turned out Syanna was not just the young lady banished from court those many years ago. No, she was also Rhena, the missing, presumed-to-be-kidnapped young woman who was Dettlaff's beloved. The witcher and company learned she had controlled the Cintrian, bidding him to steal Sangreal wine as well as the ducal jewel. She had also sent Dettlaff the names of the intended victims. The vampire needed all his strength to quell his desire to tear her to shreds then and there. He seemed a shard of ice as he set a meeting time and place - Syanna was to appear at Tesham Mutna three days hence to explain herself. The witcher found himself between a rock and a hard place. One the one hand, a vampire consumed by fury demanded he deliver Syanna to him in three days' time. On the other hand, Anna Henrietta awaited, expecting Dettlaff's head on a silver platter.