For those of you lucky enough to be in the World of Warcraft Legion Beta, today marks the start of the pre-Legion Event test. In the month or so leading up to the release of Legion, Azeroth is going to be invaded by hordes of demons from the Legion. As with past expansions, this content is intended to give players advanced insight into the changes for the upcoming expansion as well as provide a bit of content and possibly (usually) achievements to accomplish as well. Typically, any achievements associated with a pre-launch event become Feats of Strength and are no longer attainable after the pre-launch event.
Details of the test were posted to the official World of Warcraft forums by community manager Kaivax (emphasis mine):
On Thursday, June 2, starting at 4:00 p.m. PDT, we will activate a new realm in the Legion Beta. This Beta realm will be named “Level 100 Demon Invasions”, and it will be dedicated to testing the pre-expansion demon invasion event with as many testers as possible for approximately 24 hours. We expect to remove the realm after 24 hours.
We ask that all Beta testers please join us in focusing testing on the “Level 100 Demon Invasions” realm when it is available. While you may see demon invasions on other Beta realms, testing this content requires that your character remain at level 100, so we ask that you test on the “Level 100 Demon Invasions” realm only.
Please direct your pre-Legion event feedback to the Beta General Discussion forum, and please direct any feedback you have about other Legion subjects to the appropriate dedicated subforum.
Thank you very much! We’ll see you at the defense of Azeroth!
(source)
So for anyone beta testers out there wanting to get their hands on this event early, be sure to login to the new “Level 100 Demon Invasions“ realm starting at 4:00 p.m. PDT.