The Guardian was born c.12,950 B.C.E. He was an ancient Immortal of 14,962 years, who was once on search for the mysterious source of Immortality circa 7988 B.C.E. He and the others didn't succeed, and he was forced to protect The Source forever.
Duncan MacLeod and the Future
Eventually a new group of Immortals started to search for The Source. At some point after the year A.D. 2012, The Guardian beheaded Zai Jie, who had discovered its location. Then he fought with Duncan MacLeod. However, the fight was broken off by Joe Dawson, who escaped with MacLeod in tow.
He then started to hunt down MacLeod and the other Immortals during their quest for The Source. He killed Joe Dawson, Reggie Weller, and Cardinal Giovanni, using his extreme speed and strength. However, in the end, he was defeated by MacLeod during their final fight at the threshold-nexus of The Source, when the Highlander refused to strike the killing blow.
The Guardian was destroyed forever, and Duncan earned mortality as a final reward.