At the first you go to the Darkmoon Faire, which comes to town once per month, starting on the 1st Sunday of the month, and lasts for one week. And win the Darkmoon Deathmatch twelve times and complete the quest they give you. You must be at the Darkmoon Deathmatch Arena when the special chest spawns (it spawns every three hours).
Here you can buy Darkmoon Dominator for EU or US Servers.
Also you'll get guide for receiving Darkmoon Dominator