To Light a Candle, the second book in The Obsidian Trilogy from Mercedes Lackey and James Mallory

The Demon Queen Attacks!

To his own surprise, young Kellen, once the disappointing son of the great Mage who leads the City's Mage Council, has become a powerful Knight-Mage. Valued for his bravery and his skills as both wizard and warrior, Kellen joins the Elves' war councils. Yet he cannot convince the City of his birth that it is in terrible danger.
Kellen's sister Idalia, a Wild Mage with great healing ability, has pledged her heart to Jermayan, a proud Elven warrior. Someday Idalia will pay a tragic Price for a world-saving work of Wild Magic, but until then, she will claim any joy life can offer her.
Jermayan, who has learned much fighting at Kellen's side and loving the human Idalia, finds that everything changes when he Bonds with a dragon while rescuing the Elf Prince and becomes the first Elven Mage in a thousand years.
Furious at her enemies' success with the dragon, the Demon Queen attacks in force. Light struggles against Dark, like flickering candle flames buried deep in the shadow of Obsidian Mountain.