Ragnaros the Firelord is the ruler of all fire elementals in the Warcraft universe, and one of the four elemental generals of the Old Gods. Ragnaros was the first Elemental Lord to appear in the game and one of the original endbosses of World of Warcraft, being the final boss of the Molten Core raid inside Blackrock Mountain, where he was summoned by the Dark Iron Dwarves. Ragnaros' body is a massive swirling vortex of flame, a trait which has since been recycled for several other characters, and wields an enormous hammer called "Sulfuras, Hand of Ragnaros", which was one of the first "Legendary" items in the game. Ragnaros is summoned in the Molten Core by Majordomo Executus, who is then killed by his master for summoning him prematurely.
Ragnaros returns in Cataclysm (which also features the other three elemental lords), where he has been summoned again in Mount Hyjal and is working with Deathwing to destroy Nordrassil. After a long campaign of quests, he is faced by the player, along with Cenarius, Malfurion Stormrage and Hamuul Runetotem in his inner sanctum and defeated again, however the player is told that Ragnaros is still a threat and can only actually be killed inside his home dimension of the Firelands.
Ragnaros is finally faced in his home realm of the Firelands in Patch 4.2. If Ragnaros is killed here, he dies permanently. His new guardian here is Majordomo Staghelm. On normal difficulty, the final battle ends when Ragnaros is reduced to 10% health and he retreats. Only on Heroic difficulty is the true final phase of the battle available and can Ragnaros actually be killed. However, even though he was destroyed for good, it was stated his power could never be truly destroyed and another Firelord could rise up so monitoring the Firelands would still last forever.