Here's the trailer for the first truly major content patch of Legion, including Sargeras' voice!
And here are some extra details concerning the patch that Blizzard shared over twitter:
The Cathedral of Eternal Night is available immediately (after a couple of short intro quests on the Broken Shore). https://t.co/hZKEckQXgC— WarcraftDevs (@WarcraftDevs) March 23, 2017
Yes, in NA. With weekly maintenance in other regions (e.g. 29th for EU). Legion Season 3 will start at the same time. https://t.co/eyE3Aapjmd— WarcraftDevs (@WarcraftDevs) March 23, 2017