The Mask of Loki — Roger Zelazny; Thomas T. Thomas

The wizard thinks he's in control. The gods think that's funny...

It started in the 12th century when their avatars first joined in battle. On that occasion the sorcerous Hasan al Sabah, the first and Chief Assassin won handily against Thomas Amnet, Knight Templar and White Magician. There have been many duels since then, and in each the undying Arab has ended the life of Loki's avatar. But each time the avatar is reborn,and the Assassin tires....

It is now the 21st century. Loki's time approaches, and Ahriman, Lord of Darkness, must fall.