How to get

Regular Nefarian is obtained by completing the Hidden Laboratory wing of Blackrock Mountain.

As a Wild format card, both regular and golden versions of Nefarian can also be crafted for the following amounts:

Card Crafting cost
Disenchanting
Nefarian 1600 / 400
Golden Nefarian 3200/1600

Generarted Cards: Tail Swipe

Notes

• Nefarian's Battlecry generates 2 spell cards and adds them to the player's hand. These cards are chosen randomly from all collectible spells belonging to the opponent's class. The contents of the opponent's deck and hand have no effect on which cards are generated. The selection is also unaffected by card rarity.
• Nefarian's Battlecry may generate 2 copies of the same spell.
• The cards generated by Nefarian depend on the class of the opponent's current hero, even if they started as a different class.
  • If the enemy hero has been replaced by Lord Jaraxxus, this card's Battlecry will generate warlock spells, regardless of the opponent's original class.
  • When played against a player-Ragnaros, 2 Tail Swipes are generated. This contradicts official sources on what should happen (2 class cards from the original class) and is therefore assumed to be a bug. It is worth noting that 2 Tail Swipes are enough to kill the 8 Health Ragnaros hero.
  • If used against an adventure boss, Nefarian's Battlecry will generate 2 copies of Tail Swipe.

Strategy

Since Nefarian is a costly 9-drop it would be ideal if the player were to play him while having control of the board, while the opponent is low on cards, or use cards that reduce his cost. Nefarian is exceptionally good should the opposing class cover a weakness of the player's class, for example a Mage against a Warrior, as Warriors have few high-damage removal spells, and very few board clearing spells. Options such as taunt, healing, and overall control of the board will, much like other expensive cards, allow Nefarian to be played more easily. Druids can also use options such as Wild Growth in order to play Nefarian earlier. Paladins also have Dragon Consort as a means of lowering Nefarian's cost to seven. Nefarian's effect makes it unlikely that he will ever be entirely useless, and while it is possible that a player may gain two copies of Totemic Might while not playing Shaman, even cards such as Moonfire are capable of being useful.

Nefarian is also a good addition to a Yogg-Saron deck for adding more spells to use. He is also useful for Rogue decks that use Ethereal Peddler against 8/9 classes.

Certain classes are more likely to have better results than others (though this list is subjective);

Best:

• Mage, as Mages have extremely good spells such as Frostbolt, Flamestrike, and Pyroblast. Most spells are usable regardless of the situation.
• Priest, as Priests have excellent single-target removal such as Shadow Word: Pain, strong health boosts, and some good board-clearing spells, such as Lightbomb.
• Warlock, as many spells are usable, largely self-damaging ones, such as Hellfire.

Average:

• Shaman, as many of their spells are useful, such as Lightning Bolt, though some spells are borderline useless outside of Shaman, such as Totemic Might.
• Druid, as Druid has some strong buff spells, good healing spells, and moderately good single-target removal, such as Starfire.
• Rogue, as Rogue has some exceptionally good spells such as Eviscerate, though some spells may be useless outside of weapon classes.
• Hunter, with various good secrets, some good damaging spells such as Multi-Shot, few spells are likely to be entirely useless.
• Demon Hunter, a good chance for high-impact spells like Metamorphosis or Skull of Gul'dan, although there are a few potential duds.

Poor:

• Paladin, as while Paladin has excellent buff spells, there are many negligible secrets, or minor effect spells.
• Warrior, as many Warrior spells have poor efficiency, such as Mortal Strike, or simply do not do enough, such as Whirlwind.

Quotes

Druid
Druid! The wild is mine to control!

Hunter
The hunter becomes the hunted!
Mage
Mage! You should be more careful when you play with magic!

Paladin
Paladin! Does the Light forsake you?

Priest
Priest! The Light serves me now!

Rogue
Rogue! Stop hiding and face me!

Shaman
Shaman! The elements obey me now!

Warlock
Warlock! You play with magic beyond your understanding!

Warrior
Warrior! Your strength becomes your weakness!

Other
Your magic is mine!

Attack
Let the games begin!

https://www.deviantart.com/toamatanui123/art/Nefarian-Son-of-Deathwing-875563944