Moongather by Jo Clayton.

A young warrior woman, Serroi fights to keep dark magic from destroying her world, in the first book of Jo Clayton’s acclaimed Duel of Sorcery fantasy trilogy.

Serroi is unique in her world, and was nearly put to death in infancy as a result. A green-skinned “misborn,” small in stature with an unnatural connection to the natural world, she was nonetheless chosen and exceptionally trained as a meie warrior. As such, she fears nothing, except the cold and inscrutable Nor and their dark magic.

Something in Serroi’s childhood awakened her to a shocking and terrible truth about these malevolent wizards, one of whom both saved and cursed her in her earliest years. It is her deep-seated terror that causes her to betray and abandon her shieldmate, Tayyan, during a rooftop battle with a magic-wielder, a craven act that threatens to haunt Serroi to the end of her days.

However, it is not cowardice that makes her run, but rather her knowledge of a great evil in the offing. In that instant before flight, Serroi recognizes the coming of something monstrous, though she cannot yet put a name to it. Now it is up to the young warrior to somehow prevent the unthinkable: She must alter a grim destiny that is set to occur on the fateful and fearful night known as Moongather, when demons will be free to enter the world.