Looks like one of Microsoft's surprises for next week has just been spoiled. A leak on the official Xbox Store points to an unannounced Open Beta for Halo Wars 2 that will begin next week on Monday, June 13th and last for one week until June 20th. Check out the image below.
Microsoft is set to host their Press Conference on the day the Open Beta begins, June 13th at 9:30 AM PST/12:30 EST. It's reasonable to conclude that Microsoft had originally planned for the Open Beta to be a surprise, but apparently they didn't get the memo that the internet doesn't do those anymore.
If you want to check out the listing for yourself, just head over to Major Nelson's (Larry Hyrb) Xbox profile page. The beta as listed as a game that he follows. Halo Wars 2 supports online play of up to six players and there is currently no mention of Windows 10 players getting the Open Beta.
Halo Wars 2 was revealed during Gamescom 2015 for both Xbox One and Windows 10 and there have also been a few leaks on possible spoilers for the game.
Sources: [Twitter, Eurogamer.net]