As the adventure's name reveals, this is the magician Medivh Karazhan's tower, which veteran World of Warcraft players may remember from The Burning Crusade. The expansion begins in the final preparatory moments before the grand feast that Medivh planned, but of course things go to hell and it's up to you as the player to use your card game skills to save both Medivh and the party. As in previous adventures you face bosses to get through the opening prologue and the four wings of the castle.
The biggest challenge you will probably find is the building of your deck before each boss, as you can't play with the same deck through the whole game. A boss might quickly need to be taken down by hordes of cheap cards, while others require an entirely different approach. It may be fun to outwit enemies with their own crafted decks, but the funnest bosses in the game require you to get by with the cards the game gives you. One of the best examples of this is the chess-inspired boss fight that you encounter quite early on. The unique battle cards that are offered require a fairly thorough rethink of conventional strategies.
Now, if you feel that the new bosses might be too easy to defeat you can of course obviously try your luck with the "Heroic" versions of them. The harder level is certainly a challenge, and it requires a good player with the right deck to even have a chance of succeeding. If you meet the challenge, however, you will be rewarded with a new unique look to your deck that you can then brag about.
Bosses and the challenges are great, but the most important thing with a card game is obviously the cards themselves, and of course One Night in Karazhan offers new ones - 45 of them to be precise. Of these there are 18 neutral cards that can be used by all classes, and the remaining 27 cards are tied to the different classes (three cards unique to each of the nine classes). As in previous expansions/adventures you unlock all the cards as you defeat bosses and proceed through Karazhan's wings.
The multiplayer mode is extremely important in Hearthstone: Heroes of Warcraft, so all the cards must be properly balanced. Fortunately, we have not encountered any card from One Night in Karazhan that seems too unbalanced. In most cases, they only offer new gameplay for the classes and other cards work nicely as a replacement for older cards.
On the whole, One Night in Karazhan is a fun addition to an already great game. The price tag might well feel somewhat high, but then again this is only the first part of the adventure, and it's one that's clearly going to be worth it for most Hearthstone players. One Night in Karazhan is also a good entry point for all the old World of Warcraft players who want to feel a bit nostalgic for the old raids, but haven't gotten around to playing Hearthstone: Heroes of Warcraft yet.