I had never heard of this book, first of an eventual five-volume series, before I found it in the Andre Norton bibliography. It’s a collaboration with Sasha Miller, author of several fantasy novels of her own, and it came out quite late in Norton’s life, with the later volumes published posthumously. It’s essentially a Witch World hommage, not quite fanfic in that it’s supposedly set in a secondary world of its own, but the settings, characters, and world are clearly based on Norton’s iconic series.
There’s a medievalzoid realm ruled by four families—a la the Mantles of Arvon. There are Sea Rovers who are Sulcarmen with the serial numbers still clearly visible. There’s a huge, deadly Bog inhabited by a wide range of monsters and assorted clans and tribes of ugly, misshapen, barbaric people. There’s a tradition of Wisewomen, represented by the mysterious Zazar. There are ancient ruined cities everywhere, and in the first volume there’s a strong suggestion that the world is being invaded by aliens from another world or dimension.