The Stonecore is the heart of Deepholm, the elemental plane of earth. Before Deathwing was corrupted by the Old Gods, he was the Earthwarder, the Dragon aspect of earth, alongside the likes of Malygos and Alexstrasza. So Deepholm and Deathwing are well connected. Sometime before the start of World of Warcraft, Deathwing suffered a defeat at the hands of the other aspects. They presumed him dead, but in reality he had just retreated to the Stonecore, where he rested, regained his strength, and spread his influence throughout Azeroth via the Twilight's Hammer Cultists.
When he got better, Deathwing took off like a dragon out of Deepholm and broke into the mortal plane of Azeroth where he proceeded to burn everything he could set his eyes on. In the process he destroyed what we know as the World Pillar, a significant piece of planet supporting architecture that you'll spend the majority of your time questing in Deepholm to repair.
With Deathwing now gone, the Twilight's Hammer Cultists regard the place of his recovery as sacred. They will protect it with their lives, and use it to perform dark rituals in service to their lord and the Old Gods.
When you first enter the Stonecore, you'll be tasked with chasing down a Twilight cultist who is transporting some important documents. If you've quested in Deepholm at all, you'll recognize the carrier as Millhouse Manastorm. Millhouse was first seen back in The Burning Crusade as part of the final boss battle in the Arcatraz. He was a prisoner there that when released, assists you in defeating the final boss. We don't really learn how Millhouse got involved with the Twilight's Hammer cultists, but he's here now, and it's up to you to stop him. After chasing him for awhile, Millhouse decides to put a stop to your shenanigans and begins to cast the spell Impending Doooooooooom! Fortunately, you never find out what exactly the spell does because Millhouse gets eaten by Corborus before he can finish casting the spell.
Corborus is a gyreworm, similar to the many other stone worms you see throughout Deepholm. After eating Millhouse, it's up to you to stop the worm from eating you and your party as another snack.
Your next encounter is with Slabhide, a stone drake who, under the control of the cult, tries to keep you from getting closer to the inner sanctum of the Stonecore. Not much else is known about the drake, but you have to vanquish him to move further into the dungeon.
Ozruk is a stone giant, and is a child of Therazane, the Stonemother. She tells you that he did not return from the Stonecore when she called to him. She believes him to be under the power of the dark magics of the cult and requests that you return him to the earth. You do what you have to do and open the way to the World's Heart at the center of the Stonecore.
And in that heart is High Priestess Azil, who along with her hundreds of followers, is preparing a ritual of some dark origin. After putting a stop to that, you may want to take a look around the room behind her altar. This is the very place that Deathwing lay before he burst into the world above. If you recall the opening cinematic of Cataclysm, it shows the cultists reinforcing Deathwing's armor plating before he takes off. This is the place where that happened. And thanks to you, it's now out of the hands of the cultists. For now.
