Hearthstone's Eighth Expansion is The Witchwood! It launched on April 12th 2018.
135 new cards were added to the game.
A new Shaman Hero Card, "Hagatha", is seen briefly at the end of the reveal video. Ben Brode confirmed this is the only Hero Card in the expansion. (Source)
Every class will be getting 2 Legendary cards this expansion. (Source)
A new mechanic called Echo is being added to the game.
A new mechanic called Rush is being added to the game. It's like Charge but less OP.
The pre-order comes with 70 packs instead of the usual 50. You also get a card back. You can pre-order here or in-game.
A card back for completing Monster Hunt.
A legendary class card for logging in on release.
Three card packs for logging in on release.
Up to four card packs for completing daily quests in the lead-up to the expansion. (Source)
Works like Dungeon Run in that you play a hero against bosses and build a deck as you progress.
Does not use normal Hearthstone heroes, this has four unique characters.
The final boss at the end of Monster Hunt is different for each of the four classes.
Below are pictures of the Cannoneer, Houndmaster, Time Tinkerer, and Tracker heroes from Monster Hunt.
New Keyword: Echo
Echo cards can be played multiple times on the turn you play them. Each time, it’ll add a ghostly copy of the card back to your hand that disappears at the end of your turn.
New Keyword: Rush
Minions with the Rush keyword can attack other minions immediately after they hit the board, either by being played or summoned. They cannot attack heroes until the turn after they enter play, however.
Hagatha the Witch
The only new playable hero card being added to the game, Hagatha has unique emotes and a special passive Hero Power.