Every thousand years, the Zoroastrian demon Ahriman returns to wreak havoc on the Earth. He had been defeated centuries ago by an Immortal named Timothy of Gilliam. An archaeologist warned Duncan MacLeod that he was the champion chosen to do battle with the forces of evil, namely Ahriman. Duncan was skeptical, until he saw people he had killed, such as James Horton and Kronos, alive and well. Others thought Duncan was losing his mind, and when he accidentally killed Richie Ryan because he thought Richie was Ahriman, Duncan felt a deep resentment and retreated to a monastery. For one year, Duncan was missing on the Watcher's database, and even Joe Dawson couldn't find him. One year later, Duncan returned to Paris to visit Richie's grave and met Ahriman who was disguised as Kronos and Horton. Duncan vowed to destroy Ahriman. A mysterious woman named Sophie Baines knew how to defeat the demon, and although she had died, her spirit guided Duncan and, along with the help of Joe and Father Beaufort, allowed Duncan to defeat the evil demon.
The character has been perceived in a very negative light by the fans, considering him an element that was added to the show in order to boost interest after it had run out of ideas for new enemies for the Highlander. They also consider the storylines themselves do not add to the story of Duncan MacLeod, and ultimately only served as a way to cast out Richie Ryan, a popular major character who died because of Ahriman's intervention. David Abramowitz, however, creator of the idea of the supernatural demon storyline, considers the character and the storyline as "valuable stories" to the Highlander canon.