After laughing about the nature of your meeting, Regis reminds you that a powerful entity such as Dettlaff will not be found if he doesn’t wish to be. And that includes finding them with megascopes. It is fortunate, then, that you’ve kept your memento from the encounter in Corvo Bianco basement with the bruxa. After showing Regis the hand of Dettlaff, you can ask how he intends to use the hand, what the ring on the hand signifies, and why he can’t simply summon Dettlaff. Regis’ verbose responses lead you to a conclusion; a decoction needs to be brewed, too.
Regis agrees that a hallucinatory poison is needed. Alas, the ingredients are scattered far and wide. Summoning ravens to aid in finding the elements of the decoction, Regis asks if you’d like a snifter of mandrake while you both wait. You can:
Accept, prompting a lengthy conversation between you and the vampire Regis. Answer how you wish.
Refuse, and mention to Regis you’ll sleep until the Ravens find the ingredients you seek.
Some time later, the ravens fly back and confirm your suspicions that no kobolds or mamunes can be found, and therefore their organs are safe from your ingredient-collecting. Instead, a Wight haunting an abandoned residence near the Trastamara Hunting Cottage could be harvested, though you don’t believe the birds; Witchers should have culled them all. Optionally ask Regis about the Wight, the curse the Wight is under, and then agree to pay the Wight a visit (you’re attempting to collect some of it’s saliva, which should be fun). You can’t miss the place; it’s covered in spoons.