Greetings! My name is HeartOfCards and I'd like to share a deck that you are all probably quite familiar with! However id like to make it apparent that this deck has quite a few variations/possible card replacements to suit different play styles.
This variation takes the deck to a faster speed. Kel'Thuzad is a pretty weird card and lots of people dislike him for the fact that he does require a board to be useful. Ive tested the deck extensively with both cards and i switch between the two legendaries occasionally.
A fun twist on the deck, that also works quite well: Feugen and Stalagg have good base stats and can be used to bait out a silence or removal card for either themselves or on Thaddius. This potentially allows your Sylvanas Windrunner Cairne Bloodhoof, and Ragnaros the Firelord/Kel'Thuzad to pose far greater threats and unleash pain for longer amounts of time.
In the current meta we're in, I would very much disagree with this replacement, as the meta seems to be mostly filled with aggro decks with an occasional handlock or control warrior. You'll be able to get some value off of The Black Knight against hunter if he runs Sludge Belcher or Houndmaster, however I dont really think its a great right now especially since Cairne Bloodhoof has synergy with the rest of your deck and is a very difficult minion to deal with.
Closing thoughts: This deck is extremely strong, and can win against practically anything with well-drawn cards, as such it's favored among many other currently popular decks.