"The White Flame Dancing on the Graves of his Foes," a title bestowed upon Emhyr by those well aware of just how ruthless the Nilfgaardian emperor can be. He was technically introduced in Blood of Elves, but was slightly expanded on in The Last Wish which showed us that he has had his own experiences with magic and monsters. He's one of Sapkowski's greatest villains, something Hissrich clearly realizes.
She describes Emhyr as a ruthless, imperialistic masquerader. An emperor chasing power and the stability it brings as a way of escaping the demons of his chaotic past. Hissrich made it a point to name him as a villain that people would understand. Whether or not that indicates Emhyr will be the main antagonist of the pilot, we cannot say, but there's clearly a chance.
Among the novel series' greatest villains is the mage, Vilgefortz, who was introduced in Time of Contempt. He was described by Yennefer as being young but powerful. So powerful in fact that he was able to go toe to toe with a higher vampire and live. More than that. He faced a higher vampire and defeated him rather spectacularly. Vilgefortz may appear youthful but he is likely over a century old-- which is typical for experienced mages-- and in that century, he has witnessed and done things that haunt him. Instead of pushing him into a life of virtue, it has twisted him into something monstrous.
He is described in the tweet as power-hungry and ruthless, among other things. Just as the books describe Vilgefortz as being a wholeheartedly evil but notably handsome man, Hissrich tweeted, "he is the essence of evil hidden behind beauty, which makes him very, very dangerous."
Some may recognize this name from the expansion to The Witcher III: Wild Hunt, "Blood and Wine." Regis is an incredibly intelligent and well-connected higher vampire who was introduced in Baptism of Fire. Unlike most, he is exceptionally peaceful and in-tune with humanity. He's a close friend of Geralt, who often turns to the vampire for his wisdom. He wasn't always that way. Regis is over 400 years old and when he was younger, he was much crueller.
In regards to the upcoming show, Regis is described as eloquent, worldly and haunted. Hissrich described him as being the humanity personified as he is a malevolent monster turned virtuous hero-- a perfect example of the level of complexity you can expect from the show and the novels, if you choose to start reading them.