Blizzard announced that during the second or third week of August, after the pre-patch has been out for a few weeks to give players the chance to try out the changes, the Legion will begin their incursion into Azeroth.
You’ll be able to tell where current invasions are by checking your world map, and can then head to the assaulted areas to assist in the defense of your world.
Each week after will cause the Legion to intensify their assaults and more and more areas will be affected. These events are scenarios that will reward nethershards to purchase gear and transmog items. Chests can also drop during the incursions and will have varying loot, including Legion themed weapons.