Blizzard are introducing a completely new way to play Hearthstone with Dungeon Run mode - a single-player adventure that pits you against a series of eight powerful bosses. The free mode selects encounters from a pool of 48 potential bosses, meaning the possible combinations of encounters and decks that you can play run into the hundreds.
Dungeon Run is part of the new Hearthstone Kobolds and Catacombs expansion.
Hearthstone Dungeon Run bosses and encounters
The system is designed to be infinitely replayable, and the difficulty certainly helps that: if you die in Dungeon Run, the entire session is over. Some bosses are even "rare spawns" that won't trigger as often as others, and are often a little bit weirder than more standard bosses.