This article contains major spoilers for The Witcher season 3The Witcher season 3 features several epic duels, including Yennefer's unfortunate clash with Vilgefortz's powerful assistant, Lydia van Bredevoort (who managed to expose a lot of Yennefer's weaknesses). Lydia is a mysterious character in The Witcher, though the identity of her benefactor, Vilgefortz of Roggeveen, is finally revealed in The Witcher season 3's big twist. The mystery of Lydia van Bredevoort is deepened by the illusion that hides her injured face and the fact that she communicates solely through telepathy.
Despite Lydia's injuries, she shows an impressive amount of magical power during The Witcher. Near the end of The Witcher season 3, Lydia has an important confrontation with Yennefer of Vengerberg on the infamous Isle of Thanned. Yennefer is widely hailed as one of the strongest mages on the continent, and yet, she meets her match in the mysterious form of Lydia van Bredevoort. Lydia handles Yennefer with apparent ease, demonstrating an immense level of power and prompting speculation as to how she become so powerful.
Vilgefortz Made Lydia More Powerful Than Yennefer (But How?)
It's apparent that Vilgefortz made Lydia more powerful, but his methods remain a mystery. Yennefer seems extremely confident before their duel begins, but Lydia claims her master has given her power beyond Yennefer's, which is evidently true based on the lopsided fight that ensues. If not for the intervention of Triss Merigold, it seems Yennefer would have been killed. With that in mind, whatever Vilgefortz did to Lydia, it clearly gave her an incredible level of power, which makes one wonder just how powerful Vilgefortz himself must be.
As to Vilgefortz's methods in making Lydia stronger, it's possible he gave her a powerfully enchanted amulet. Vilgefortz has a history of giving powerful amulets to his servants in the books, most notably, Rience, so it would make sense if that was how he managed it. Alternatively, it's possible that Lydia was already powerful, and Vilgefortz simply taught her new ways to use her power that made her more dangerous. Either way, Vilgefortz ability to give his servants such immense power makes his own power a daunting prospect.
Why Couldn't Lydia Use Her Powerful Magic To Fully Fix Her Face?
In the books, Lydia's face and larynx were injured when Vilgefortz insisted she participate in the analysis of a cursed artifact. The artifact essentially exploded with magical energy, destroying Lydia's jaw and larynx in such a way that they stubbornly resisted all attempts at regeneration. In The Witcher show, her face is injured when examining a witcher mutagen that contains Ciri's blood, but the magical nature of the injury seems to prevent regeneration the same way her book injury does.
Magical injuries often do not respond to attempts at healing, even when the mage in question is as powerful as Lydia or Vilgefortz. Lydia's injury came in service of Vilgefortz, so he also likely tried to heal it and failed. Coincidentally, Vilgefortz ends up suffering a similar injury in The Witcher season 3, and his does not seem to be healable either.