Baron Rivendare was once a wealthy landowner of the kingdom of Lordaeron. Being a close friend to Kel'Thuzad, however, he chose to follow he friend on the pathway to damnation and became a Death Knight of the Scourge.
After the kingdom fell to the Lich King's horrific creations, Baron Rivendare was in charge of watching over Stratholm, an important base of operations for the Scourge. There, he ensured that Kel'Thuzad's citadel, Naxxramas, was well protected with magical Ata'mal Crystals.
The heroes of Azeroth did not take this kindly, and the Argent Dawn sent a small group of adventurers to storm Stratholm and slay the Baron and his minions.
With the Baron slain and the crystals destroyed, Naxxramas was vulnerable for attack.
Years later, Rivendare was resurrected and was now in charge of the Four Horseman, the four most powerful Death Knights of the Scourge. During the War in the Frozen Wastes, Naxxramas was once again assaulted, and Rivendare and his Horsemen were slain in battle.