Crossing the Threshold - The Silver Branch, #1

Sir Medraut, prince of Orcades, died protecting Queen Guinevere after King Arthur's final battle, and Liadain's heart shattered. The world changed as ancient powers waned, mortal ambition drowned mortal virtue, and men chased thrones and kingdoms at any cost. Only a handful of Camelot's knights chose to keep their watch through the long years, defending what was left of Arthur's dream. Centuries after Camelot became but a fairy tale, Liadain inghean uí Gwynn, daughter of the High King of the Otherworld, longs for death to free her from her immortal grief.
Remote, untouchable, alone, she waits for the end of her days. But a summons from the chief bard of Camelot places Liadain once more in the company of Arthur's faithful knights, on a path that leads to the promise of dreams reborn. If she can survive the quest. If she can face the past. If she remembers what it means to love. The epic tale of the greatest knights of the Round Table racing to defeat an ancient enemy transports you to a time when magic warred with honor, and great loves defeated the powers of darkness.
Legendary names of old, and deeds worthy of legend.