He is what some would call the lesser known of the triumvirate that led the eredar of Argus prior to Sargeras' intervention. Certainly he lacked the camaraderie of Kil'jaeden and Velen -- he had little to do with the grudge match Kil'jaeden carried, nor did he particularly care where Velen had gone, or what he was doing with the eredar who chose to turn down Sargeras' offer. What he was, however, was far more deadly in comparison. With no grudges to distract him, Archimonde was easily one of Sargeras' most effective military commanders -- cunning, incredibly powerful, and deadly.
Archimonde's tale stretches two different points in Warcraft's history. The first, thousands upon thousands of years ago, before Azeroth's continents had split and settled into the familiar placement we see on maps today. The second, not so long ago at all -- and his appearance leveled a city, brought together and united a world divided in hatred, and ended the precious gift of immortality given to an ancient race long secluded and hidden away.
Please note: The following Know Your Lore contains a few small spoilers for Warlords of Draenor.