Hearthpwn

Overview:
Abuse the auto hero power each turn with aura minions and double Thaddius.

Mulligan:
Mulligan for a Corrupting Mist, it's a major component of the win condition. You should also aim for one of the Tar minions: Tar Creeper, Tar Lurker.

Early Game:
The match is very easy to exploit, but you still have to be careful, as Thaddius can deal an enormous amount of damage to you quickly. Use Corrupting Mist on Stalagg and Feugen when they come out, because their deaths will count toward your own Thaddius.

If you have a minion or spell to attack with, attack Stalagg, because after the next swap he will have the higher attack. After that, play the Tar minions, and each turn, their buffed stat will become permanent after the swap.

Mid Game:
Use spells and minions to trade, but don't let your Tar minions die. When you draw your own Stalagg and Feugen, kill them as quick as you can, and each will give you Thaddius.

Late Game:
Now it's just a steady game of keeping control and going face.

Video:

My other Curse of Naxxramas decks:
Arachnid Quarter
     Anub'Rekhan (Taunt Warrior)
     Grand Widow Faerlina (Odd Warrior)
     Maexxna (Dragon Priest)

Plague Quarter
     Noth the Plaguebringer (Spell Hunter)
     Heigan the Unclean (Control Shaman)
     Loatheb (Control Priest)

Military Quarter
     Instructor Razuvious (Taunt Druid)
     Gothik the Harvester (Mill Druid)
     The Four Horsemen (Resurrect Priest)

Construct Quarter
     Patchwerk (Freeze Mage)
     Grobbulus (Odd Patron Warrior)
     Gluth (Divine Shield Paladin)
     Thaddius (Control Warlock)

Frostwyrm Lair
     Sapphiron (Fatigue Mage)
     Kel'Thuzad (Mill Rogue)