Seems like we are going to hear from it very soon (not tm)! Some people received an upload notification for it on the official youtube channel and one user even managed to screen record it. I really didn't expected to hear from it so soon, but if anything that makes me even more hyped!

source

https://www.reddit.com/r/hearthstone/comments/pamjko/i_was_able_to_record_the_mercenaries_leak/

Edit:

AND IT'S LAUNCHING OCTOBER 12TH!

https://www.reddit.com/r/hearthstone/comments/pamdeq/got_this_just_now_in_my_youtube_subscribed_list/ha5k6q2/?utm_medium=android_app&utm_source=share&context=3