Hearthpwn

After I got Kel'Thuzad, I wanted to create a deck where he would come to good use, and this was the result. 

The deck has two main goals: Clear the enemy board by making beneficial trades and get card advantage. It uses lots of secrets and beneficial deathrattles. In the early game, focus on buffing up an Undertaker with Leper Gnomes, Loot Hoarders or Unstable Ghouls. Since I put two of each in the deck, chances are you'll have a few in your starting hand, and when your opponent plays some minions, you can put down Baron Rivendare, clear his board, and you'll do at least 4 damage per Leper Gnome and draw 2 cards per Loot Hoarder. The trade is beneficial to you because your minions are most valuable when they die (thus also justifying the damage Unstable Ghouls do to your own minions). Alternatively, you could summon a Cult Master and draw even more cards. 

The reason I use quite a few secrets is because I most of the time can play them for free. The Mad Scientists (which by the way can buff your Undertaker too) will pick a random Secret from your deck and put it on the board. Combo this with Baron Rivendare and you get two Secrets for free. This is not only effective because you save some mana/time but also because it thins out your deck, increasing the probability that you will draw the cards you need. Likewise, Kirin Tor Mage lets you cast a Secret from your hand without paying its cost. As if that wasn't good enough, Ethereal Arcanist becomes a major threat to your opponent when you have Secrets out. At worst, it's a 5/5. If it survives one round, it's a 7/7 (possibly damaged, of course). 

Later in the game, see if you can combo Baron Rivendare with a Sludge Belcher or two. When Sludge Belcher dies, it will summon two 1/2 Slimes with Taunt. The combo is really strong, since you essentially get 5 Attack and 9 Health out of one card that costs 5 mana. Great, right? Now try comboing this... with Kel'Thuzad. 
I know, I know, it sounds unlikely, but it has actually happened to me more often than one would think. I have watched my enemies killing my Sludge Belcher, letting it summon 2 taunting Slimes in the process, only to see it respawn when the turn ended. I was litterally lol'ing. Also, keep in mind that when Kel'Thuzad reanimates your minions with Deathrattle, Undertakers will get another buff!

The problem with combo decks is, that if your opponent removes your minions from the board too early, you'll never get to utilize their full potential. That's why I put Counterspell in the deck, and while it's a great Secret, my favorite has to be Duplicate (thanks a lot, Naxx!). I would love to be able to put another Ethereal Arcanist in, but I don't know what I should take out, and Duplicate solves the problem. If you can time it correctly, your opponent will remove a threat from your board, and BOOM! You've got two more coming. 

Ususally you'll force your opponent into spending too many resources on removing threats on your board. Counterspell protects your minions, and Duplicate, well, duplicates them when they do die, thus giving you the card advantage for the late game, when your opponent only has one or two cards in their hand. That's when you summon Kel'Thuzad or Ragnaros the Firelord and go for the kill. 

Almost any deck needs removal, so I put two Fireballs, two Polymorphs, a Flamestrike, and Ragnaros the Firelord in there too. 

I did not really expect much from the deck to begin with. Back then, I had Stalagg and Feugen in it too, but I found them too inconsistent. It was too big of a risk to include them, though a Stalagg-Feugen-Baron Rivendare-Kel'Thuzad-combo sounds... completely f*cking OP. 
It turns out, though, that the deck worked much smoother, than I had expected. But there may still be a few tweaks to make. 

Let me hear what you think. Try it out, give it 5-10 games, and leave a comment with suggestions. That'd be great, thanks!

     - Lauge99

Changelog:

I crafted Cairne Bloodhoof and replaced Spellbreaker with him.