I'm pretty sure Vilgefortz turned Regis into a puddle of molten slag.it is a writing flaw, really. i should be able to tell him very cruel and indecent things. lol
I also hope that, the vampire regis, being old and seemingly enhanced, managed to survive and is in tw3. hes my favourite character of the series. They kill all the good ones.
Addendum: @Kad_Venku makes some amazing points in his longer post with the spoiler tag. I 100% agree that the original story with Geralt freeing Duny from the curse was likely not tied to the whole Emhyr/Ciri/prophecy bit. It fits in with the whole "grain of truth" set of fairy tales in The Last Wish perfectly, but is not even remotely congruent with the rest of the story. I don't think poor translation is the cause either, I simply doubt Sapkowski meant to tie the original story into the larger one. In The Last Wish, Duny seems to definitely hold Geralt in the highest regard while showing arguably legitimate feelings for Pavetta despite the law of surprise compelling him; keep in mind Pavetta agrees when Geralt calls the qualifying portion of Duny's claim into finalizing the oath, stating she must agree to fulfill the law of surprise or Duny's claim would be nullified.
I haven't read Lady of the Lake, I didn't think the English translation was released yet, but that far in the series I would find it highly likely that Sapkowski simply applied triage to the script and made Emhyr's character expose his role as Duny to be an altruistic farce. Much easier for a character to deny something about themselves that happened in their past when it comes to chronology in writing, or a cheat code as I call it (No offense to Sapkowski, but he even admits he didn't come up with the material that resulted in The Witcher, I forgot the writer he took it/was inspired from, easily searched for though; perhaps someone here knows).
Anyhow just throwing my own observations up in contrast to some interesting theories and a great perspective offered by @Kad_Venku.