Kazzak

"I remember well the sting of defeat at the conclusion of the Third War. I have waited far too long for my revenge. Now the shadow of the Legion falls over this world. It is only a matter of time until all of your failed creation... is undone."

-Kazzak

Kazzak was one of the demon lords that Archimonde brought with him during the attack on Azeroth. Kazzak was left behind after the remaining forces of the Burning Legion fled. Lord Kazzak was introduced into World of Warcraft in patch 1.3 as a boss. When the game's first expansion, The Burning Crusade, was released, Kazzak apparently got a promotion to Doom Lord after reopening the Dark Portal, and relocated to the Outlands.

On Azeroth
When the demonic Burning Legion invaded the world of Azeroth for the 2nd time, Archimonde, field commander of the Legion, brought with him his top lieutenants. One among them was Lord Kazzak. During the battle at Mount Hyjal, Archimonde and most of his lackeys were killed. The only survivors of this battle were Kazzak and Pit Lord Azgalor. Azgalor fled Azeroth soon after, leaving Kazzak in control of the remaining Legion forces on Azeroth. Kazzak made his main base of operations in the region known as the Blasted Lands. Unfortunately, Kazzak was completely cut off from other Legion members on Azeroth. Even if he could, they would most likely avoid him in fear of his great power. Eventually, he found an ancient relic that would allow him to accomplish his goal - to reopen the Dark Portal. He put the relic to use, and opened the portal that lead to the shattered world of Draenor, now known as the Outlands.

In The Outlands
When he opened the Dark Portal, he left minion, Highlord Kruul, in charge of the Legion on Azeroth. Kazzak was promoted to Doom Lord after opening the portal, and set up his new main base of operations on a mountain known as the Throne of Kil'jaeden. He deployed his minions, Arazzius and Vorakem, to lead the Legion forces on the ground. During the war against Illidan Stormrage, the heroes of Azeroth learned of Kazzak's location, and mounted an assault on the Throne of Kil'jaeden. Kazzak was heavily guarded by the Legion, but his guards were slain, and Kazzak and the heroes battled. Kazzak was defeated, but his it is unknown wether he survived or not.