Hearthstone (opens in new tab) pro Dima "Rdu" Radu dropped a big spoiler during a stream today when he revealed that World of Warcraft headliner Sylvanas Windrunner (opens in new tab) is coming to Blizzard's CCG as a new Hunter hero skin.
As a streamer, all content on his account is unlocked—even, apparently, stuff that isn't released yet. That's what leads to the "slip," seen in the clip above: After taking a quick look at the new Druid hero Dame Hazelbark, Radu turns the page and, for just a few seconds, highlights the Sylvanas hero listing, which isn't active but is unmistakably there. Flipping away, he says, "Spoilers," before giggling nervously.
This wouldn't be Sylvanas' first appearance in Hearthstone, as she's also in the game as a legendary neutral minion card (opens in new tab). But the card was moved to the Hall of Fame in 2017, meaning that it's no longer available in Standard format and cannot be acquired through card packs.
Our best guess is that Sylvanas will be available as a paid cosmetic, although there's nothing else known about it at this point, including when she might arrive. I've reached out to Blizzard for more information and will update if I receive a reply.