The Massacre of the Khalar vezhven was an event in Vaes Dothrak in Essos that took place concurrently with the Liberation of Slaver's Bay, in which Daenerys Targaryen massacred the entirety of the Khalar vezhven and claimed leadership over all Dothraki from Vaes Dothrak.
After Daenerys emerges from the burning hut unscathed, the Dothraki bow before her and accept her as their ruler. While Daenerys flies to Meereen on the back of Drogon, the Dothraki follow on their horses. During the second siege of Meereen, the Dothraki and Daario Naharis arrive before the city's gates and swiftly slaughter the Sons of the Harpy who are executing some of Meereen's inhabitants.
In the books
As Daenerys' storyline in the sixth season has preceded her book storyline, this event has not yet appeared in the books. Although Daenerys has been found by a khalasar, she has not yet been brought to Vaes Dothrak.