Originally Ragnaros was the first main boss from Classic "Vanilla" WoW from Molten Core, inside Blackrock mountain's own Blackrock Depths where u deal with the Iron Dwarves. No original patch trailer thoe, but here's the patch 4.2 "Rage of the Firelands" instead.
https://www.youtube.com/watch?v=HsXglDs81FU

Name: Ragnaros
Faction: Fire elements
Race: Fire element
Background: The Fire Lord
Bio:
Ragnaros, the Fire Lord, was used by the titans to create the world of Azeroth along with the other elements, but he didn't aprove of them using him and sealing his kind away in the firelands. As years past, the Dark Iron Dwarves used a spell to summon Ragnaros into their world and to be bend to their will agains the other clans. In the events it created Blackrock Mountain, but utterly failed as the Molten Core served as his domain deep underground in Blackrock Mountain. Recantly, the adventures entered the Molten Core and defeated Ragnaros. Years gone by and during the Cataclysm, Deathwing manage to bring back Ragnaros to Azeroth once again, but with one task, to burn the world tree Nordrassil. But the Defenders of Hyjal manage to beat Ragnaros elements straight back to the Firelands where he too fell to the druids and the heroes of Azeroth.