'Shadow of Obsession' by Rebecca Neason

There can be only one...

He is immortal. A Scottish warrior born four hundred years ago. He is not alone. For centuries he has fought others like himself. He can die only if a foe takes his head, capturing his life-force in an event known as the Quickening. But his battles are eternal...for in the end, there can be only one. He is Duncan MacLeod.

The Highlander

Man of Peace

Once the Immortal Darius was a brutal warrior and the terror of Europe. Then outside the gates of Paris, a transformation took place, and Darius became a saintly man of peace. His teachings touched many lives-including the Highlander's. Now, to honor his friend's memory, MacLeod protects Victor Paulus, a mortal who carries on Darius' legacy. But Darius left another legacy-a thwarted ex-lover who spent centuries plotting against Darius, who has sworn to detroy anyone he called a friend. Starting with Victor Paulus and Duncan MacLeod...

Notes:
*Featuring Duncan MacLeod