The Ice Witches are a women-only order of spellcasters in Kislev specializing in ice-based magic, which include Tzarina Katarin and the first Tzarina of Kislev Miska the Slaughterer. Ice Witches recruit magically gifted girls from Kislev, and while they primarily recruit the daughters of nobles they will take in (or pass on to another Ice Witch) peasant girls with magical abilities.
They have a very strong connection to the people of Kislev, especially the Gospadars as it was the Ice Witches who led them to Kislev and are considered Kislevs guardians, although technically the Ice Witches protect the Ice magic that blows through Kislev as it is connected to the land and requires the land to be intact for it to be preserved. Their origins are a not truly known however the Gospadar tale for them says that while the Gospadars were divided clans, and in the process of getting the shit kicked out off them by the Kurgan and Hung for not taking the Chaos-pill, a priestess prayed to their gods until one actually answered. This goddess had no real name and was mostly known as "The Widow" or "Kislev". This spirit/Goddess/passing Daemon looking for a laugh, told the priestess of a far off land of ice and tundra where the Gospadar would be saved, and gave her the power to "control winter itself". With the Widow whispering in her ear the priestess started to unite the Gospadar, becoming the first Khan-Queen of the Gospadar clans and started to teach Ice Magic to other priestess and shamans of other clans to further bulk out the number of ice flinging bitches in the Gospadar ranks. After generations the Widow eventually led the current Khan-Queen (who was almost certainly Miska, but the tale doesn't actually say) and the Gospadars to the land that would become known as Kislev. And the rest is the Gospadar invasion of Kislev and the ass kicking of the Ungols, Ostermarkers and Ostlanders. The accuracy of this tale is dubious but, legends and myths aren't retold because they are realistic or accurate.
While the Gospadars love the Ice Witches for being their savours and foundation of their society, the Ungols are less fond of them as the Ice Witches will usually take Ungol resources, by force if necessary, when ever they deem fit and they seem to be one of the few remaining points of contention between the two groups of people (the Ice Witches probably do the same to Gospadars but the Gospadars are probably far happier to give the Ice Witches what they want). Though the Ungols do widely like Katarin.
Since the Ice witches have been part of Kislevite society since the countries founding, the order has a political and cultural sway over the population and nobles that would make the Catholic church red with envy, and commonly use this power to diminish and silence cults and groups that threaten their power, while bolstering and protecting others that aid their interests. As previously stated the Ice witches are an all female order of witches, and Kislev has an overall ban on men with magical powers and will root out and kill any (local) men displaying magical powers. Most families who discover that their son is a wizard will usually smuggle him to the Empire so he can be trained and saved by the college of mages in Altdorf. This rabid manslaughter (Ha) of male wizards isn't just to satiate some weird murder boner either, as their is prophecy of a male Ice Wizard bringing ruin to Ice magic and tainting it forever so the Ice Witches used their influence and power to make sure that the entire population hate the idea of male wizards and that they root out and kill them to ensure the prophecy never passes.
A new edition to Kislev, added in Warhammer: The Old World and debuting in Total Warhammer III. They are the body guard of the Ice Witches who have blessed them by channelling the winds of Ice through their weapons to give them a frosty edge. They are very good fighters and like other Kislevite forces are trained to use both bows and hand weapons in battles. Think the Sisters of Averlorn/Sisters of the Thorn, but Slavic and LARPing as Jadis from Narnia.