As with all Hearthstone adventures the first wing will launch August 11th, the second the week after and they will continue to release at that pace until the final wing launch. This adventure is different in that they are offering everyone a free prologue mission that will unlock two free cards for everyone! So even if you don't plan on picking up the whole adventure, playing through the free mission for the two cards seems like a pretty good deal!
I've scoured the web for more details about the expansion so if you are interested keep reading!
If you are interested in watching Hearthstone designer Ben Brode talk about the new adventure in the recent ChinaJoy twitch stream, you can watch it below.
https://www.youtube.com/watch?v=4w5Yaqrkszk
Redditor Nostalgia37 is keeping a spreadsheet up-to-date with all the Karazhan cards as they are unveiled by Blizzard. So far 9 of 45 cards have been revealed and they all look like they will have a pretty big impact on the meta. As the Karazhan gets closer to launch i'll be talking about the cards I am most interested in .
https://www.reddit.com/r/hearthstone/comments/4v60mk/the_marvelous_prerelease_reveal_chart_all/
PC Gamer has a interesting playthrough of one of the missions of One Night in Karazhan. I recommend muting the audio.
Thttps://www.youtube.com/watch?v=g7e_Udn88E8

Ok, I will definitely be reinstalling Hearthstone on my phone to check this out :D
That sounds good, looking forward to hearing what you think of it!