Who is Nivellen?

I have played the Witcher 3, read the first 3 novels, and seen the first season of the Netflix series. I am unfamiliar with the character of Nivellen, who is shown in the new trailer for the Netflix series. Ithought season 2 was based off the third novel (Blood of Elves) so I’m unsure how I don’t recognize the character. Did I miss something or is he from the fourth novel?