Val Griswold Ford: 2 books

Rose is a True Soul, a pure human with magic uncorrupted by the demons who have taken over since Lucifer defeated Yahweh in the final war and claimed the Earth as his own. Living on borrowed time, she accepts a dangerous mission from a member of the Resistance: getting a member of the nearly extinct human royal family out of the city before they are both offered as sacrifices.

During the ten days surrounding the Solstices, no magic can be used to kill. This puts a serious cramp in Nikki Jeffries' (aka the Horseman Death) style, since a lot of her magic is based in death. Breaking the Covenant isn't an option. So when the StarChild sends her to the North Pole to find a missing...