TL;DR:

The goal of this deck is to rapidly complete The Demon Seed questline with the many cheap self-damage tools available to Warlock, play Blightborn Tamsin on curve, and then use your various self-damage effects to burn your opponent down.

NOTE: This is a theorycrafted deck. This guide is a preliminary analysis and does not reflect any data or experience with the deck.

Deck Overview

This deck is entirely built around The Demon Seed, whose final reward causes your self-damage to instead damage your opponent. Earning Blightborn Tamsin requires 21 total points of self-damage - 6 to complete the first step, 7 to complete the second step, and 8 to complete the final step. The wide array of self-damage tools printed for Warlock over Hearthstone's history - combined with neutral standout Crystallizer - make it possible to achieve this as early as turn three, allowing you to play Blightborn Tamsin on curve. Darkglare lets you do this more consistently while putting a large amount of stats on the board, potentially including a heavily-discounted Flesh Giant.

After playing Blightborn Tamsin, your hero power and nearly every card in your deck transforms into burn.  Additionally, combining your cheap minions with Unlicensed Apothecary allows you to deal high amounts of burst damage early in the game. It's entirely possible to play Blightborn Tamsin on turn 5, and then deal 30+ damage to your opponent on turn 6 by combining Unlicensed Apothecary with your 1-drops or discounted Flesh Giant.

Given how reliant this deck is on self-damage, conserving your life total is absolutely essential. For this reason the deck includes a handful of spells that restore your health while helping you control the board.  A huge chunk of your damage is going to come from post-quest battlecries, so don't be afraid to trade to give yourself breathing room.

Card Choices:

• The Demon Seed - The reason you're playing this deck. Always keep it - you have little hope of winning without it.
• Raise Dead - Excellent resource generator and one of the most efficient ways to rack up self-damage. Having this card on turn 4 makes it much easier to play Blightborn Tamsin on curve.
• Crystallizer - One of the best cards in the deck. Progresses your quest by 5 without costing you any health, and post-quest becomes a 1 mana 1/3 that heals 5 while damaging your opponent for 5. If you could run 29 copies of this card, you would.
• Flame Imp - One of the all-time great 1-drops in Hearthstone. Double Flame Imp on turn 2 is a lot of stats and instantly progresses your quest.
• Kobold Librarian - Another one of those cards that's been in just about every Warlock deck since its inception, and it's at its best here.
• Spirit Bomb - Four damage is a lot for one mana. Great for keeping the board clear and the four self-damage pairs nicely with your smaller sources of self-damage.
• Touch of the Nathrezim - Incredibly efficient card. Cheap healing and removal are limited in Warlock, and this card will often do both.
• Bloodbound Imp - 2/5 is an excellent statline for 2 mana. I've generally avoided including delayed self-damage in this deck, but this card is likely to stick on the board and can go a long way towards enabling Darkglare.
• Darkglare - Though its effect won't activate once the quest completes, this deck's abundance of cheap self-damage allows Darkglare to enable some bonkers turns.
• Drain Soul - Warlock's ubiquitous healing spell. Not an ideal early game play, but it's efficient and you'll likely need lifegain.
• Unstable Shadow Blast - When dealing damage to yourself is the goal of the deck, this card suddenly looks much more appealing. The ability to target your own minions gives some nice flexibility.
• Backfire - Efficient card draw that deals 3 face damage post-quest. It's generally not ideal to use this before completing the quest, but it's nice to have the option.
• Ornery Tortoise - A 3 mana 3/5 isn't particularly impressive these days, but it's good enough. The 5 self-damage nicely complements your various smaller sources of self-damage and is a good source of burst post-quest.
• Unlicensed Apothecary - The deck's biggest source of burst damage, and a great OTK enabler against decks with a lot of healing - if the stars align, you can OTK as soon as turn 6. Burst potential aside, this card is also great to play on turn 3 in a pinch. A 3 mana 5/5 is excellent, and if it sticks you'll likely be able to finish your questline on turn 4. However, this approach can lead to a lot of wasted self-damage so keep an eye on your quest progress.
• Pit Lord - As with Ornery Tortoise, Pit Lord's statline isn't particularly impressive. The big chunk of self-damage is nice though, and hey - if Pit Lord is ever going to see play, it's going to be in a Demon Seed deck. Give the big guy a chance!
• Flesh Giant - Nearly every other card in the deck changes your health during your turns, so the cost of these will go down quite quickly. Your self-damage cards won't further decrease the cost after playing Blightborn Tamsin, but completing the quest will almost always reduce their cost to 0 (In rare cases the armor from Crystallizer will cause them to cost 1). You'll almost always be able to play one on turn 4 in a pinch, but it's usually best to play them when they cost 0. Playing a giant with Blightborn Tamsin to set up for lethal or as a part of a combo with Unlicensed Apothecary can be game-winning.

Potential Strengths:

• Can finish the quest early - A good opening lets you play Blightborn Tamsin on curve, and even with bad RNG it won't take much longer than that.
• Efficient early-game minions and life gain can help stave off minion-based aggro while doubling as burst late-game.
• Traditional control tools are less effective, as big chunks of damage are front-loaded into battlecries post-quest.
• Unlicensed Apothecary enables OTKs in combination with any other minions in your deck - no fringe support required!
• Darkglare and Flesh Giant can enable some great early tempo plays and help complete your quest quickly.
  
  

Possible Weaknesses:

• Very narrow - Outside of an exceptional early Darkglare/Giants turn, the quest is the only real win condition.
• Aggressive decks may bring your HP too low to complete the quest, destroying your win condition.
• Vulnerable to fast decks with significant burst damage from-hand.
• The curve is exceptionally low. Though a low curve is essential for getting the quest online quickly, more draw or a slightly higher curve may be needed.
• Healing is limited, and some of the most enticing options (Lesser Amethyst Spellstone, Deathweb Spider) are costly and cannot be activated once Blightborn Tamsin has been played.
• Will likely require tinkering to hit the appropriate healing | damage ratio for the Stormwind meta.

Possible Inclusions:

• The Soularium: If draw feels lacking, this is your best choice. Since the deck's curve is so low, it doesn't take long for this to essentially be 1 mana draw 3.
• Vulgar Homunculus and Death's Head Cultist: Barely missed the cut. The taunt can soak up some damage, but there are better sources of healing/self-damage and a 2/4 taunt isn't the most impressive statline. If you find your HP getting too low too fast, these are options to consider.
• Wrathguard: Aggressively statted and has a high self-damage potential, but is dependent on how your opponent responds to it. Worth considering though - even if it takes a while to get the self-damage from it, unless it gets transformed you still have a 2 mana 4/3.
• Hooked Reaver: A 4 mana 7/7 taunt is undeniably powerful - and given the Questline, you will reach 15 health or lower almost every game. That being said, it is at the top of this deck's curve and may end up delaying quest completion for too long. I felt that cheap healing was better suited to this deck's gameplan than beefy taunts, but it's worth trying out.
• Chittering Tunneler: The ability to discover a spell can give you some real flexibility in how much self-damage you take.  A cheap spell can eliminate wasted damage, while a big one can complete an entire quest stage by itself. The spell itself can be good to have too, given the low curve of this deck. However, Warlock has a lot of bad spells so this may end up not having enough of a payoff.
• Felfire Potion: Playing this card with the quest reward active turns it into a 5 damage board clear bundled with a Pyroblast. I think the high cost and symmetrical effect make it a sub-optimal choice for this deck, but it's too cool of an effect to not consider.
• Lesser Amethyst Spellstone: Healing is very valuable in this deck, and this card is a very solid choice when upgraded. However, once you've played Blightborn Tamsin, you will no longer be able to take self-damage and thus will be unable to upgrade the spellstone.  Given how quickly we're aiming to finish the quest, this card seemed too inconsistent considering how inefficient the base version is.
• Cheaty Anklebiter: This card might not look like much, but health is valuable enough of a resource that it could find real success in this deck. Not only does the win condition involve dealing damage to yourself, you also will always be skipping turn one in order to play the quest. That makes a lifesteal ping sound pretty appealing, though that will of course depend on what form aggressive decks take. Additionally, since extra self-damage doesn't carry over to future stages of the quest, being able to ping yourself for 1 could be valuable.

Possible Exclusions:

• Darkglare: As mentioned previously, effects that trigger on self-damage won't happen once you've played Blightborn Tamsin.  This means that after playing her, Darkglare essentially becomes a 2 mana 2/3 with no text. This makes it by far the worst card in the deck while Tamsin's effect is active, but I believe the explosive early game potential makes up for this.
  
  

Tips:

• Extra self-damage does not carry over to future stages of The Demon Seed. If you're at 5/6 on the quest and then deal 5 damage to yourself, you will end up at 0/7 on the next part.
• Aim to play your cards in a way that minimizes this wasted self-damage! Though this deck has life-gain tools, too much wasted health may make it impossible to complete the quest - and play most of your cards, for that matter.
• Your hero power can be a great way to minimize wasted self-damage. Though using your hero power early in the game can feel slow, doing so will sometimes allow you to maximize the efficiency of your self-damage.
• You complete the quest by "taking damage on your turns." You don't have to use your own cards to do this! An enemy's Leper Gnome will progress your quest if it dies on your turn, as will triggering secrets like Explosive Runes and Explosive Trap. When these cards are played against you, use them to your advantage.
  
  

Possible Deckbuilding Directions:

• Combo/OTK: With Blightborn Tamsin activated, Unlicensed Apothecary transforms from a rarely-seen meme card into a one-card OTK engine. Gone are the days of elaborate Treachery setups! Combining Apothecary with token cards like Fiendish Circle and cheap symmetrical board clears like Rain of Fire can allow you to deal a significant amount of damage in one turn with only a handful of cards.
• Fatigue with Neeru/Hemet: Hemet, Jungle Hunter and Neeru Fireblade are a natural combo. Blow up your deck, play Neeru the next turn, easy! If you make your deck spell-heavy, you can even run Free Admission as a pseudo-tutor.  However, Neeru on its own is rarely enough to kill your opponent before you fatigue out. A completed quest solves this problem by letting you stall with an endless flood of imps as fatigue kills your opponent, while also allowing for burst damage opportunities with Unlicensed Apothecary. It's a meme, sure, but some memes are worth exploring.
• Control: I don't think there's much of a future for The Demon Seed in control decks, but it's worth noting that such a deck would allow you to include more control/survival tools while progressing the quest with a few heftier sources of self-damage such as Riftcleaver.  Attacking into enemy minions with the Lord Jaraxxus 3/8 weapon can be a nice source of damage. Regardless of how late you activate the quest, if you're able to drag the game out into fatigue you will almost certainly win. However, I think more traditional control Warlock decks will be more effective than one specifically built around The Demon Seed.