The demon Archimonde started life as a member of the High Council of the Eredar, a powerful, magic-using race inhabiting the world of Argus. Along with his fellow Council members, Kil'jaeden and Velen, Archimonde was contacted by the Dark Titan, Sargeras, who promised them great power if they chose to follow him. Archimonde and Kil'jaeden, along with many other eredar, accepted, being turned into demons and joining Sargeras' Burning Legion, while Velen, warned in a vision, fled Argus with any who would listen.
Archimonde and Kil'jaeden soon became Sargeras' most powerful commanders, and they accompanied him to the world of Azeroth, seeking to use the Well of Eternity to take over the world. During the War of the Ancients, Archimonde killed the powerful Ancient, Malorne, but the Legion was repulsed when the Well imploded, destroying the demon's portal into the world.
Another invasion attempt was made through the Legion's new minions, the Undead Scourge. Archimonde himself came to Azeroth again, seeking the power of the World Tree. Although it at first appeared Archimonde might achieve his goal, he was killed when the Archdruid Malfurion Stormrage summoned the spirits of nature to destroy him in a last-ditch ambush.
Battle vs. Chernabog (by CuchulainSetanta)
No battle was written.
This was a rather effortless win for Chernabog. He was basically the devil, and was able to curbstomp Archimonde.