From the blurb: "Kyllan the warrior, Kaththea the untried witch, Kemoc, whose powers could surpass all others - these are the half-Earthlings, half witch-brood family menaced by the sorceries of an unknown enemy.
The burden of the struggle fell to Kemoc, who was forced to summon his untested powers in the battle to match the alien evil threatening the Witch World"
I highly recommend reading the preceeding books in the series before you read this one.
"Warlock of the Witch World" Andre Norton
I /think/ this is a part of a series, but I can't find any reference to other books. It opens in the middle of the story. A young man with newly discovered supernatural powers has to travel through a hellish landscape to save his sister.
If you are a fan of Andre Norton, this is a must read in the Witch World series. Norton is a master story teller and this book is no exception.