Molten Core Ragnaros

New player here. How do I summon Ragnaros in Molten Core? I tried googling it and it said to get the quest from Duke Hydraxis in Azshara and take a reward item to douse the flames in Molten Core in order to summon another Boss and finally Ragnaros. However, in WOTLK Warmane, there is no such quest that rewards that item.

Was wondering the same thing. Is it still necessary to douse the runes?

You have to kill every other boss. After you've killed every boss, Majordomo is going to be waiting for you where Ragnaros spawns. Talk to him to start the fight.