This is the follow-up to Dungeon Run in the newest expansion. It was released just a couple days ago (about a week after the expansion release).
It's good. Better than Dungeon Run, in my opinion. As with Dungeon Run, it's free. I play fairly little Hearthstone now, but I'm glad I stuck around for this content.
It fixes up some of Dungeon Run's flaws. The final bosses are actually fair. The treasures and passives seem a bit better balanced (strong ones weaker, weak ones stronger).
Instead of playing the 9 base classes, you pick from 4 special classes just for this mode: Tracker, Cannoneer, Houndmaster, Time-Tinker. The cards are collectible cards, but the hero powers are new. The design of the hero powers to me signals a new direction for Hearthstone: acceptance of higher complexity. In particular, the hero powers for Cannoneer and Time-Tinker are very challenging to use optimally. The thing I enjoy most about Hearthstone is planning out a good play in a turn, and those two classes strongly emphasize that task.
Once you finish a run with all 4 classes, a Final Challenge opens up. I enjoyed it a lot.
The difficulty level is lower than Dungeon Run. I won first attempt with a couple of the classes, and beat the Final Challenge on first attempt too. That said, I don't think it's too easy, since you need to think a lot about your plays to do well. But replayability might not be as good as Dungeon Run.