Executus by Kraxius

At the top of the elemental hierarchy, just beneath Ragnaros himself, resides Majordomo Executus. This flamewaker gained his exalted status by proving to be nearly invincible in the battles that raged within the elemental plane. Rumor has it that Executus supplanted Baron Geddon, and the two have been rivals ever since. Although this rumor has yet to be confirmed, there is certainly no doubt that Ragnaros does not tolerate failure. Thus, the lesser flamewakers (and, of course, Baron Geddon) eagerly await a misstep by Executus, anticipating their chance to vie for the coveted attention of the Firelord.[1]

Executus will only be spawned after you have defeated all the other bosses in Molten Core (except for Ragnaros who is summoned after Executus's defeat). Pre 3.0.8, you also had to douse the runes using Aqual Quintessence. You obtained the Aqual Quintessence by gaining Honored status with the Hydraxian Waterlords, and completing the quest chain ending with Hands of the Enemy.

Executus comes with four Flamewaker Healers and four Flamewaker Elite guards making this a total of a nine enemy encounter. Executus will "submit" and the fight is won when all the adds are dead. He then appears in Ragnaros' chamber and will summon the Firelord to begin the encounter.