Azeroth
Azeroth is the primary "World of Warcraft" continent in which many races were born, including elves, gnomes, trolls, goblins, tauren, dwarfs, humans, dragons and more. Over its time, Azeroth has frequently become the site of conflict, undergoing three great wars before emerging with three strong, major powers: the Horde, the Alliance and the Scourge. Even through the destruction leading to Azeroth falling, these three powers continue to fight, vying for ultimate control over the lands.
Deathwing
Deathwing was originally a character from "Warcraft II," and later made a cameo appearance during the original version of "World of Warcraft" as a human named Neltharion. Since then, he had transformed into a dragon and fallen into a deep slumber, where he had not stirred in years. However, he was awoken, his power magnified immensely through the Old Gods' influence. This caused Deathwing to literally erupt into the world of Azeroth.
Cataclysmic Events
Deathwing's entrance into Azeroth shook the land wildly, exploding into the world and causing nature to react violently. Azeroth felt the shake of earthquakes, floods and even volcanic eruptions. Powerful faction leaders fell to their graves, shaking the very foundation upon which they were created. The end result was the significant change to how Azeroth was shaped.
Aftermath
With the exception of most races' starting zones, all parts of Azeroth were affected by Deathwing's onslaught. For example, a lava fissure separated the Barrens into two sections, the Thousand Needles were completely flooded, the Kingdom of Gilneas was opened by the destruction of the Greymane Wall, and the Wetlands also faced heavy landscape changes. To accommodate for the changes, Blizzard also completely overhauled the main storyline so that it matches the destruction that occurs all around the characters.