When Darkness Falls

When Darkness Falls, the third book in The Obsidian Trilogy from Mercedes Lackey and James Mallory

Despite a great working of Wild Magic and High Magic that struck at the heart of the Demon Queen's evil plots, Knight-Mage Kellen and his Elven allies are still seen as enemies by the human Mage Council. The Elves and their allies must find a way to shatter the Demons' hold on the human Mages, for without their High Magic, the forces of Light will be destroyed by the forces of Darkness.
The Commander of the Armies of Light decides to turn an Elven mine into a refuge for those driven from their homes by the war. Kellen is put in command of the force of engineers and warriors who will fortify the mine and does not learn until it is nearly too late that the caverns are full of Shadowed Elves. Vestakia, the half-human, half-Demon healer, finds that she has begun to be able to read the mind of her Demon father, Crown Prince Zyperis.
The Demon Queen, Savilla, is preparing a great sacrifice that will summon He Who Is back to the world and make her the ultimate ruler forever. When Wild Mage Idalia learns of Savilla's plans, she knows there is only one way to prevent the coming of He Who Is. A very different sort of sacrifice must be made.