Remember what Hearthstone was like at launch, when Blizzard's card game was all the rage? That original 240 card set is coming back in a new way to play the game as it was in 2014.
Classic Format is Blizzard giving Hearthstone the WoW Classic treatment. As detailed in a post on Battle.net, Classic Format lets you craft decks and compete using Hearthstone's original 240 cards as they were in 2014.
When Classic format launches with the patch for the next expansion (so, before the expansion content itself unlocks), your entire collection of Basic, Classic, and relevant Hall of Fame cards will be mirrored and available for deck building in the Classic format, as they were in the 220.127.116.1132 patch of June 2014 (this is after Blizzard made "a few important changes" following the March 2014 launch). All balance changes since June 2014 are reverted.
This means Warsong Commander can give minions Charge, Holy Smite can go face, and Leeroy Jenkins costs four mana. This goes both ways: Demon Hunter wasn't in Hearthstone in 2014, so it will not be part of the Classic format.
While all balance changes for these cards over the years will be reverted, cosmetic changes will remain. So, Felstalker remains Felstalker and will not revert to Succubus.
A handful of us at Eurogamer played Hearthstone obsessively when it came out (and before, during the beta). I fondly remember playing every day, fussing over tempo, working out the maths behind the game, trying (and usually failing) to climb the online ladder. I did once beat Trump (not that one) though. That match is in the video below:
I fell off Hearthstone as it grew in complexity and other games emerged, so to see a Classic format hit the game is hugely exciting. Time to dust off that Consecration!
Expect to hear more during BlizzCon on 19th February.