The Diamond Throne (The Elenium Trilogy, Book 1)

Book One of the classic ELENIUM series.
After a long exile, Pandion Knight Sparhawk returns to his native land to find his young queen grievously ill.
Ehlana has been poisoned and will die unless a cure can be found within a year. The life force of twelve of her sworn knights is all that sustains her; but one knight will be lost within the passing of each month if the antidote isn’t found.
To save his queen, his comrades, and the stability of the kingdom, Sparhawk begins the search for the cure, only to discover a greater and more pervasive evil than he could ever have imagined.