During the early part of the Second Age the element of Darkness as a political force had something of a - well I would call it a Golden Age if that wasn't a bit Light based - partly as a result of Arkat defeating Chaos while associated with Darkness. This included the rise of two powerful Darkness states among humans, the Stygian Empire in Ralios and the Spolite Empire in north western Peloria.
Officially, the Spolites believed in maintaining a balance in the world among the different elements, and turned to Darkness in order to balance the Light based Dara Happan empire. Witches were their Priestesses. They were ruled by an Emperor of which there were seven, after which the Spolites were conquered by the Carmanian empire.
It is not clear what they were actually like, as their destruction was quite complete and accounts of their empire survive only in the records of their enemies. The actual connections between the original early Second Age Spolite Witches and the late Third Age Yolena also isn't clear. She may be deriving her inspiration as much from the propaganda as genuine traditions...
As to their magic, it isn't clear what they used other than "Darkness". The term "witch" appears in other contexts as a civilised society's word for "Shaman(ess)", as in "Jakaleel the Witch" for example, but it may just be a Spolite title. While Yolanda is using sorcery that is probably a result of a milennium of Carmanian rule: There is no known native sorcery tradition that far east before the Carmanians arrive. They are most likely, given the location, to have been Theist, but spirits are also possible.
If you do want grimoires for them (nothing is impossible, and they might have picked it up from Arkat or one of his followers) they would contain elements of the nasty spells, but they would have much more than that. You don't build a nation on nastyness alone. They were a significant empire for seven generations. they will have had various grimoires on various topics.