K'zrath, Plague Lord of Madness
His entire win condition is to buff C'Thun and play him for basically an instant win. Although he is still fully capable of beating you without doing so. His hero powers are all designed to mess with your minions one way or another, so you need to prepare your deck for what’s to come. In general, this Plague Lord is somewhat okay to run zoo decks against, since most of his removal only affects 1-2 minions. There are a few exceptions though; Vanish, Psychic Scream, and his third hero power, but the first 2 don’t occur very often.
His first hero power is one that you can actually sometimes work in your favor. Since you can occasionally gain control of his minions, you can strategically trade to where you steal his strongest one. He does however know how to arrange his minions from weakest to strongest going left to right, so it can be tough. Being able to give him one of your weaker minions while maintaining a strong board can also be difficult.
Your rightmost minion has a chance to attack into something and die, making the one next to it what K’zrath steals from you. It’s best in this stage to have several minions on the board as often as you can, and having your board going from left to right organized from strongest to weakest.
His new form of consistent removal continues to involve minion placement. It’s even more important now since in heroic mode, his minions aren’t affected by this. Not only do you have to consider the Health of the minions you’re playing, you need to consider who your placing them between. On top of that, you still have to take into account that your rightmost minion will attack into something and may end up dying as a result. While you may want to control only one minion by the end of your turn, note that his other removal tools include __ and __, which are very effective ways for him to eliminate your single target threats.
The best way to handle this is to keep something weak you don’t mind dying on your far left and a big minion that can take it to its right. It can be a pretty tall order to keep it up every turn, but if you can only get 2 minions on the board that will kill each other, it’s usually better just to play nothing. Luckily, his deck at this point is mostly C’Thun cards, which are nothing but regular minions aside from buffing C’Thun himself. It’s a little bit easier to catch back up against K’zrath than other Plague Lords since he doesn’t have any real ways to cheat out lots of tempo. If you’re capable of doing so with powerful cards or treasures, this is the stage to do it.
Upon reaching his last 100 Health, he transitions from focusing more on buffing his C'Thun to maximizing the value of his hero power. He’ll start playing cards that summon tons of small minions to abuse his hero power in order to get rid of all of your stuff. It’s at this point where his C’thun can easily win him the game even if it’s a fresh game and it’s at low attack. He’ll just place it on the right so it can attack immediately.
The best way to use your resources at this point is to ignore his big minions like Splitting Festeroot and Eccentric Scribe. If you have more minions than him, he’ll usually want to trade into your stuff rather than go face. This will a lot of times leave him with several smaller minions and most if not all of yours destroyed. This is where you clear away as much as you can with AOE damage to avoid any repercussions from his hero power. Then reload the board as best you can. You can slowly chip away at Health if you can minimize the value he gains with his hero power.
Hero Pre-Adventure Selections
Reno Jackson: There is a lot of chaotic mechanics in the fight against K’zrath, making being able to control the situation rather difficult. Amateur Mage can help you remove exactly what you want it to. You can also use it on your own minions during the 2nd stage to control which of your minions attacks. The Gatling Wand is a great treasure choice since K’zrath’s hero powers tends to screw with your minions quite a bit. It also works as a win condition if you try to buff the damage enough to be you OWN version of C'Thun.
Sir Finley: Having the power to fill up the board in this fight is valuable. The only real way K’zrath can get rid of them all in one move is with Psychic Scream. But as long as he has at least one minion, he won’t see the board as threatening enough to use it. And during the third stage, it can help you fill your board with silver hand recruits that can absorb the destroy effects of his hero power. The Amalgam minion summoned from New "Recruits" can also be a way to manipulate how his first and second hero power effect you, since it will be summoned on the far right and always be a "lowest health" minion.
Elise Starseeker: If you feel confident that K'zrath has a C'thun incoming, a board of your biggest minions can be a good way to tank the damage. Druidic Teaching is a decent way to heal off some of the damage inevitably taken from the end of turn attacks.
Brann Bronzebeard: Even though he'll usually be able to get rid of one or two of your minions, you're still gonna have to play some to keep pace with his own. Jr Excavator can help you with discounting your minions so you can catch up with his usually decent board.
Glyph of Warding: This is the perfect passive to take, as it will prevent him from playing his most powerful minion entirely. There is no reason no to take this in the 2nd Chapter when preparing to take on K’zrath.
Primordial Bulwark/Recycling: The most likely way you would die to K’zrath is by his C’Thun’s Battlecry followed by his end of turn attack. These two passive treasures can keep you alive and give you an extra turn to turn the tides.
Rocket Backpacks: The end of turn attack in this chapter can really prove to be a beating. Rocket Backpacks can help you make the trades you need to keep from being snowballed.
Crawling Claw: The chance being able to steal K’zrath’s C’Thun alone makes this treasure a good card to snatch up if you can. It works even better if you have ways to resurrect or copy it.
Bauble of Beetles/Canopic Jars/Mummy Magic: The biggest reason K’zrath’s C’Thun is so deadly is that it gets an extra attack when he ends his turn. And his Battlecry tends to be able to clear your entire board, allowing him to go straight for your Health. These treasures can help you by letting you have minions that can absorb the blow even after the board-clearing Battlecry.
Crusty the Crustacean: If you happen to survive after K’zrath plays a rather large C’Thun, you can throw it right back in his face by killing it with Crusty. Make sure that Crusty is on the far right and the board is clear so that he can deal massive damage to K’zrath when you end your turn.
Hearthstone: This may be a pretty nifty treasure in general, but it happens to be a uniquely good choice against K’zrath’s deck. Being able to reset the stats on his C’Thun when it gets too big can be a lifesaver.
Alchemist’s Stone: It’s hard to argue with discounting your hand repetitively. Having the ability to play the number of minions you need without having to worry about mana is a huge asset in a minion-centric fight.
Do Not Take
Any Weapons: K’zrath will cast Plague of Madness multiple times throughout the fight. This will break any of your valuable weapons you may have drafted. This is especially bad for Truesilver Lance, which usually has infinite durability thanks to Bubble Blower, and Aegis of Death, which will immediately kill you.
Scroll of Nonsense: This treasure is useful for keeping the early game under control, or rushing down the enemy hero with damage to the face. K’zrath however has plenty of Health and no significant early game advantages. His minions on their own are not a big deal. C’Thun minions are generally just plain in terms of stats and their only real effect is pretty much just buffing C’Thun. There aren’t really gonna be any priority targets the Scroll of Nonsense will need to help you kill until it runs out of steam.