The Culling of Dalaran was an act carried out by Jaina Proudmoore that included imprisoning the blood elves within the city of Dalaran and executing those who did not surrender. It was prompted by the theft of the Divine Bell from Darnassus and sparked the War for Dalaran.
After the establishment of Lion's Landing, King Varian Wrynn sent Anduin Llane Wrynn and to negotiate with Jaina Proudmoore the withdrawal of blood elves from Dalaran as they claimed fealty to the New Horde and represented a major threat to the war effort. Jaina Proudmoore refused; citing a desire to maintain Dalaran's neutrality. She stated that the Kirin Tor had a legacy of abuse and though members of the Kirin Tor had loyalties to both the Alliance and Horde, she hoped that they could trust each other to remain united in their mission to use power responsibly.
Theft of the Divine BellEdit
The Divine Bell, an ancient mogu artifact found by the night elves of Darnassus, was kept in the night elven capital for fear of its power should it have fallen into the wrong hands. Jaina Proudmoore assisted the night elves in creating magical barriers to prevent its theft - but despite her efforts, the Horde successfully infiltrated Darnassus and stole the artifact.
When Jaina returned to Darnassus to determine how her shields failed, she found evidence that implicated the Sunreavers and their use of Dalaran portals to aid in the theft. Furious at this betrayal, Jaina mobilized the Kirin Tor, Grand Alliance, and the Silver Covenant to purge the Horde from Dalaran.
The leader of the Sunreavers, Aethas Sunreaver, refused to order his people to leave peacefully. Jaina Proudmoore encased him in a block of ice and began her purge, marching through the streets of Dalaran flanked by water elementals. Those that surrendered were imprisoned in the Violet Hold while those that resisted were put to the sword. Chaos ensued, and after much blood was spilled, all of the blood elves in Dalaran were imprisoned in the Violet Hold.
After the purge, Jaina reported to King Varian that Dalaran was free of Horde interference and pledged the Kirin Tor to the Alliance. Varian was disappointed that Jaina had not conferred with him before she acted as he had been in secret talks with the blood elves in attempt to bring them back into the Alliance. Her attack on the Sunreavers would consequently drive the sin'dorei back to the Horde. Jaina is supposed to have told Varian to not go "soft" before she left to prepare the Kirin Tor for the War for Dalaran.
Destruction of the Divine BellEdit
Shortly after the purge, Varian discovered that his son and an Alliance champion went out on their own to stop Garrosh from using the Divine Bell. Though Anduin succeeded in stopping Garrosh destroyed the Divine Bell, it came at a great cost: Garrosh had assaulted Anduin and left the boy crippled before he could escape. Varian mournfully tended to his son while Jaina Proudmoore swore to him that the Kirin Tor would make Garrosh pay.
Adapted from Wowpedia.