This quest is not available in game.
Kill 1 Fire Lord, 1 Molten Giant, 1 Ancient Core Hound and 1 Lava Surger, then return to Duke Hydraxis in Azshara.
<name>, perhaps it is time to give you a real test. Are you ready?
At the bottom of Blackrock Depths is a passage to the Molten Core. This is a stronghold of our enemy, held by the most powerful servants of fire on this world.
Go to the Molten Core, <name>. Show me you have the strength and the will to defeat what you find there.
And if you have allies among your people, then rally them. Nothing short of an army can hope to enter the Molten Core and live.