You can feel yourself getting nervous, anticipating the struggle that is this upcoming conversation. He has leverage over you, and he absolutely knows it. If he truly could find Joannus, and there is anything he wants from you, you would be hard pressed to deny it.
"My Lord," you say, your tone growing serious. "I drank your wine. I toured your castle. We have exchanged pleasantries. Now, I must insist on the other thing I came here for."
"Your sister," Strahd says. "Joannus."
You nod.
"Your sister is in Krezk," he says.
Wait, what? Why such a simple answer? Was it really that easy?
"Ahh Joannus... you really take after her, you know," he continues. "She might be hiding in the mountains with that pitiful Abbot. Yes, I am convinced that's where she is - there is only one way she could have left, and I would have found out about it if she did. I do hope we meet again. I can arrange for my carriage to take you there immediately if you wish, but do not feel rushed to leave."
It could be that you read him all wrong. Beyond the curses and the immortality, is he just some guy in his castle who likes history, and has a creepy obsession with a woman who looks like his lost love? Or is there something you've missed?
He looks at you expectantly, waiting for the answer to his request. A subtle smile is on his face, unreadable in its purpose.
Suggestions:
>There is still business you have here in Castle Ravenloft before you leave. (specify)>Accept his request. He kept his promise. You will take a carriage to Krezk. You're coming for Joannus.>Deny his request. You will return to Vallaki first, then decide whether to head to Krezk once you've considered it enough.>Express uncertainty. You don't trust him - this is way too easy. Tell him as much, and ask to know if there is a catch. (Charisma)>Insist that the tour isn't over. Ask him to take you to another room of the castle. (specify)>Repeat your request to help him write a letter to Ireena - convince him that it really is a good idea to receive your help. (Persuasion or Deception, DC 20)>Ask him if you can roam the castle unsupervised. (Persuasion, DC 20)>Pretend to take your leave, but actually roam the castle unsupervised. (Stealth, DC 20)>Attack Strahd.>Flirt with Strahd. (Charisma, DC 20)>Ask questions. (specify)>Use Channel Divinity (1/1) for Turn Undead, or Radiance of the Dawn. (https://pastebin.com/chuTtwgW)>Cast a spell or use an item. (list: https://pastebin.com/chuTtwgW)>Other (specify) 