This quest is not available in game.
Travel to Blackrock Depths and slay Bael'Gar.
You only know that the giant resides inside Blackrock Depths. Remember to use the Altered Black Dragonflight Molt on Bael'Gar's remains to capture the Fiery Essence.
Return the Encased Fiery Essence to Cyrus Therepentous.
I wish to taste the searing flames of the mountain of fire. To let the molten fires of Blackrock cascade across my being. That essence, that searing heat, it is what I crave.
Bael'Gar resides in the Depths of Blackrock: A being of pure flame, born of the fires of Ragnaros. Slay Bael'Gar and from his smoldering remains bring for me a piece of his inner fire.
Use the molt on his remains to safely extract the inner flame.
Succeed and be rewarded.