Black Sun in a reference to a certain eclipse, better known in the context of the Curse of the Black Sun, or Mania of Mad Eltibald. It was a prophecy made by the mage Eltibald that foretold the end of the human civilization in the hands of sixty girls born during or after a certain eclipse. It features in the short story The Lesser Evil where Geralt and the wizard Stregobor have differing opinions of it.
"But of course. Except that it was called the Mania Of Mad Eltibald after the wizard who started the lark caused dozens of girls from good, even noble, families to be murdered or imprisoned in towers. They were supposed to have been possessed by demons, cursed, contaminated by the Black Sun, because that's what, in your pompous jargon, you call the most ordinary eclipse in the world."
"Elitibald wasn't mad at all. He deciphered the writing on Dauk menhirs, on tombstones in the Wozgor necropolises, and examined the legends and traditions of weretots. All of them spoke of the eclipse in no uncertain terms. The black sun was to announce the imminent return of Lilit, still honoured in the east under the name of Niya, and the extermination of the human race. Lilit's path was to be prepared by "sixty women wearing gold crowns, who would fill the river valleys with blood".""Nonsense," said the witcher. "And what's more, it doesn't even rhyme. All decent predictions rhyme. Everyone knows Eltibald and the council of wizards had in mind at the time. You took advantage of a madman's ravings to strengthen your own authority. To break up alliances, ruin marriage allegiances, stir up dynasties. In a word: to tangle the strings of crowned puppets even more."(...)
First the wizards hunted down the girls, killed them and performed autopsies when possible. After a while, when they admitted that recognizing the girls was hard, they opted to simply lock them up into towers, to isolate them. That plan suffered, because there was a brief fancy of saving beautiful girls from towers among princes. One after the other, though, the girls either perished or managed to flee their towers, and grew mighty enough for the wizards to leave them alone or get them in other ways.
It might be that the Curse became a self-fulfilling prophecy, for some of the girls who managed to flee the persecution later inflicted cruelty on others because of the treatment they had suffered.
Girls said to be afflicted
In The Price of Neutrality premium module
In this premium module for The Witcher, Deidre is Eskel's surprise child, born under the Black Sun and prophesied to ruin the world and the sister of the nobleman who wants to make sure she will not have the crown. Sabrina wants to kill her as well to examine her body and discover the truth about the Black Sun - being The Witcher, Geralt has to choose with whom he sides.
Renfri (Kinga Ilgner) in The Hexer TV series