As soon as you hit Level 82, you are eligible to accept a quest that teleports you to the center of Azeroth, where the former warchief of the Horde, Thrall, is attempting to stabilize a massive maelstrom that has opened in the sea.
Thrall, a shaman who gave up his position as warchief in order to answer the Earthen Ring's call to duty, tells you that when Deathwing erupted from the elemental plane, he damaged the World Pillar that separated the world from the plane, allowing elementals to enter Azeroth at will. He also tells you that if the World Pillar in Deepholm isn't stabilized, Azeroth might collapse through the maelstrom.
You board a gryphon to fly into the maelstrom, and enter Deepholm.
Your first stop is the Temple of Earth, which contains the Stonecore, where Deathwing waited in exile for years before erupting into Azeroth.
The Earthen Ring has taken over the ground level of the temple, and has discovered that the World Pillar is missing three segments, which need to be returned, or the world above will collapse. Guess who is going to do the legwork on this one?
You are told that the first fragment was being flown to the temple by the Horde, but discover that the ship was forced down. After investigating, you find bomb fragments that suggest that the Alliance was responsible...but after investigating the nearby Alliance airship, and finding the entire crew dead, and Twilights Hammer members roaming the decks, the true culprits are revealed.
You take a prisoner and use some "enhanced interrogation techniques"...
to find out that the Twilights Hammer has secured the fragment with a dragon for safekeeping. You lure the dragon into a fight, kill it, and recover the fragment and bring it to the temple.
The second fragment is in possession of the Earthen dwarves in the southwestern part of Deepholm. They are engaged in a furious war against the stone troggs of the area, and reward you with the fragment in exchange for your aid.
The stone giants who roam throughout Deepholm have the third fragment, but are very suspicious of mortals. When you ask this particular giant for the fragment, he responds by kicking you in the chest and launching you halfway across the zone.
You eventually earn the trust of a few of the stone giants, and are granted an audience with their ruler, Therazane, who is suspicious, but agrees to give you the third fragment if you help her kind drive out the Twilights Hammer from Deepholm, and promise to leave immediately afterward. (Also, she is really cool to you considering that you probably killed her daughter.
Naturally, you are able to clear the area of Twilights Hammer members, and you are given the final fragment of the World Pillar to set things right. You even gain the complete respect of Therazane and her subjects, and they allow you to come and go to Deepholm as you please, and give you a series of daily quests that can provide valuable rewards.
Evidence of Deathwing is prominent in Deepholm, including the area where he fell years prior on his way into exile.
The upper portion of the Temple of Earth houses the Stonecore, where Deathwing recuperated and waited out his exile. Twilights Hammer members fused heavy metal plates onto his body to hold his fiery body together. Although he is gone, they have remained behind, as they feel the Stonecore is sacred ground.