MATHES:
https://playhearthstone.com/en-us/blog/21591736/
Hearthstone Patch 10.4 - Special Arena Cards, Witchwood Pre-order, New Brawls, 20 Free Card Packs - News - HearthPwn
https://www.hearthpwn.com/news/4440-hearthstone-patch-10-4-special-arena-cards
Designer Insights with Kris Zierhut: Upcoming Arena Changes
https://www.youtube.com/watch?v=apVLfBniYLw
Arena Updates
We’re trying out a new Arena format to make choosing cards more interesting and skill testing.
New Draft Rules: Each pick in an Arena draft will now feature 3 cards of relatively equal power level, but mixed rarities.
Picks 1, 10, 20, and 30 will continue to have a guaranteed Rare quality card or better.
The increased chance to draft cards from the most recent expansion has been removed until the launch of The Witchwood.
New Arena cards chosen at BlizzCon are being added to the Arena for a limited time.
All cards that are excluded from the Arena draft pool remain unchanged.
All the other existing Arena rules in place that affect appearance rates are unchanged.