The Hidden Land — Sarah Isidore

In a time of magic and savagery...
A deadly plague rips across the Celtic lands, and to appease the gods a rare black cat must be sacrificed. But Veleda, a young girl born of a line of Druids, frees the cat, a magical creature named Mau, and follows it through a dazzling jeweled forest to the magnificent temple of Bast, the ancient cat-headed goddess of Egypt.

A goddess bestows mystical powers upon a young woman --
The startled Veleda has been chosen by the goddess to receive special gifts and powers that will end the killing scourge and heal her people. But these newly bestowed gifts will change Veleda's destiny forever; for Julius Caesar leads the legions of Rome unchecked across the continent, and the Celts are his next intended prize.

Who may be her people's only salavation
Far worse, these implacable conquerors are protected and assisted by Bast's dark sister, Sekhmet, the lion-headed goddess of blood and death. Blessed with the wisdom of the oak and the powers of Egypt, only Veleda can unite her wild and warlike peoples in order to repel the invading Roman forces.