Fregenal

From Witcher Wiki

Fregenal was a druid before he was excommunicated. He created the first koshchey, and led the band of mercenaries known as Koshchey's Men.
One of that group, a bobolak by the name of Kehl, betrayed Fregenal and helped Visenna and Korin destroy the koshchey.

Fregenal met his end, apparently, in the hands of Mikula.

He features in the short story "Droga, z której się nie wraca" (A Road with No Return), as well as in the graphic novel based on it.