The Gate of the Cat

When an Earth woman travels through a hidden portal to the magical Witch World, she joins a witch and a warrior in an epic battle against evil.
 
Kelsie McBlair has always loved the Scottish Highlands, and assumes responsibility to care for the flora and fauna therein. But when she tries to help an injured wildcat, she takes a fall between two great stones, knocking herself unconscious. She awakens in a world she never thought possible—a world where magic users, mighty heroes, and terrible monsters dwell.
 
When she comes across a dying woman, she doesn’t know what to do. And the feeling only worsens when the woman hands Kelsie a strange glowing jewel many dangerous creatures are after. Soon she finds herself on an incredible adventure accompanied by a cynical witch who doesn’t trust her and a young warrior sworn to protect her.
 
All three will be bound to each other for survival, as they are the only ones who can stem the tide of the Dark forces ravaging the land, and Kelsie—the strange Earth woman—proves to be the only one who can face off against the Lord of the Dark himself . . .
 
In The Gate of the Cat, Andre Norton’s “many Witch World fans will welcome this return to a land of powerful witches, mail-clad warriors, and mysterious Old Ones” (Library Journal).

The Gate of the Cat is the 3rd book in the Witch World: Estcarp Cycle, but you may enjoy reading the series in any order.