Ragnaros the Firelord is one of the Elemental Lords that served the Old Gods until his imprisonment by the Titans. The Titans banished Ragnaros along with the other Elemental Lords and there minions to the Elemental Plane. Once trapped in this Elemental Plane the Elemental Lords went to war with one and other to stake claim to there own part of this plane. It was during this time that Ragnaros consumed most of Prince Thunderaan in an effort to become more powerful. (This why Ragnaros looks the way he dose in game.)
Fast forward to about 300 years ago and the War of the Three Hammers. It was during this time that the Dark Iron Dwarves lead by Thaurissan, attempt to summon a powerful minion to help turn the tide of war in their favor. Not understanding fully the forces with which they were playing with, Thaurissan and the Dark Iron Dwarves summoned Ragnaros back to Azeroth. The power released by Ragnaros returning to Azeroth was so strong that it killed Thaurissan and many of the Dark Iron Dwarves and shattered the Redridge Mountains. A raging volcano now known as Blackrock Mountain and both the Searing Gorge and the Burning Steppes were made from this tramatic upheaval. The Dark Iron Dwarves that survived are now Ragnaros's slaves and live in what is now known as Blackrock Depths.
Ragnaros currently resides in the belly of Blackrock Mountain in the zone of The Molten Core. Ragnaros is the final boss in the zone. There are 9 bosses that watch over him while he tries to recover his strength from returning to Azeroth. In order to summon Ragnaros the raid must defeat Majordomo Executus. In order to summon Majordomo Executus the raid must douse the 6 runes that are guarded by 6 of the bosses using Aqual Quintessence.
The Aqual Quintessence is given by Duke Hydraxis in Azshara, once you have reached honored with the Hydraxian Waterlords faction. You may only carry one Aqual Quintessence at a time, so you will need 6 people in the raid with Aqual Quintessence to be able to summon Majordomo Executus
Magmadar (Guards a rune)
Gehennas (Guards a rune)
Garr (Guards a rune)
Shazzrah (Guards a rune)
Sulfuron Harbinger (Guards a rune)
Golemagg the Incinerator (Guards a rune)
Majordomo Executus: Behold Ragnaros, the Firelord! He who was ancient when this world was young! Bow before him, mortals! Bow before your ending!
Ragnaros: Too Soon! You have awakened me too soon, Executus! What is the meaning of this intrusion?
Majordomo Executus: These mortal infidels, my lord! They have invaded your sanctum, and seek to steal your secrets!
Ragnaros: FOOL! You allowed these insects to run rampant through the Hallowed Core, and now you lead them to my very lair? You have failed me, Executus! Justice shall be met, indeed!
Majordomo Executus dies.
Ragnaros: Now for you, insects. Boldly you sought the power of Ragnaros! Now you shall see it FIRSTHAND!