The Mirrors of Castaway Time (Bk 2, The Eye of the Stallion Series)

Sonoria, the Queen of the Thrangs, is fighting for her life against an impossible enemy:
.....as the flames leaped upwards, it seemed that instead of the twenty torches the men had lit, there were forty, and then a hundred, and then more. Sonoria turned around, and then around again. Mirrors had been set next to each torch wide, multifaceted mirrors that hung on chains and turned in the wind, reflecting the blazing light in a thousand directions, igniting the top of the hill with red-yellow fire. And in these mirrors, among the flashing, spinning light, she saw glimpses of herself, wild-haired, tall, and gaunt, gripping a sword in one hand and a shield in the other.
Then there was someone else reflected in the whirling glass. A warrior approached her. She turned toward his image, but the mirror turned with her and he disappeared. Then he was there again, behind her. She spun around, her sword slicing the empty air.