Hearthpwn

this is in serious contention for worst card in Hearthstone - I mean, it is basically a demonfire where you trade +1/+1 of stats against the flexibility of damaging a minion - which is already arguably worse and on top of that you are giving your opponent: Draw and play Wild Growth for free ...

This is good as a "teach noobs bad cards card", but as a spoiler, honestly, what were they thinking?!

I have found this card offensive. It insults my intelligence.

Quote from Royal_Naga jump

Quote from Quirriff jump

While I find this card not as bad as others i would change it to the following:

Give a friendly minion +3/+3 , at the end of your turn and destroy it if it's not a demon. your opponent gains a mana crystal.

so a worse power overwhelming for non-demons but permanent for demons.

So you solution for a bad card is make it worse?

No!, pay attention...It would mean that non-demons get the buff as well but they die at the end of turn like PO....That way it's not restricted to just demons.

While a huge drawback, I think people forget what it means to be a warlock. They channel destructive power, but it costs them. Sure, it helps the opponent, but it also helps you. /shrug.

Would be interesting to see if Demonfuse works on the Hero Jaraxxus (Lord Jaraxxus that has replaced your Hero). If it does work like this, I wonder if the + 3 Attack would be given onto the weapon, or onto Jaraxxus himself (so even if you run out of a weapon, you still have 3 Attack). Then, would the Health component leave your hero with a maximum of 18 Health? I feel like the value would far exceed the downside in this case, especially if you can get the second Demonfuse on him, making him a 3/21 (and + 6 Attack naturally), for a total of 9 damage per swing while you have a weapon.

Could be quite powerful to buff a Mistress of Pain, as it becomes a 4/7, which is difficult for a priest to remove, and 7 Health is already somewhat tough to deal with. Not to mention you would be restoring 4 Health to your Hero every single time she did damage, which could easily negate the damage taken from Life Tap and other sources.

People have been saying that this opens up opportunities for the opponent to easily remove the buffed minion. But then you can bait out removal far early in the game, which could allow you to have a better chance in the late game. Of course, the extra mana crystal on the opponent would be a pain, but that's the give/take relationship of a warlock.

And you just caused him to use 1-2 premium removal on a Voidwalker/Flame Imp. Meanwhile your Wilfred/Mal'gains/what-have-you has that much more a chance to stick. It's a deceivingly well balanced card, but will be dependent on the meta. If we're back to the hyper-agro, it's an extra point to face which is significant. If it's a control meta then it really depends on what turns 4-6 look like for everyone else.

 The card is not THAT bad,  it can be used in the mid-late game in a zoo deck (turn 8+?) as a +3 +3 discounted and then you can play some other creatures. Well, maybe it is that bad. But I think it is leagues better than poisoned blade.

Better than it looks, hits the board as a 3/3 charge.  Blessing of kings gets played and this is half the price for 3/4 the value with a condition that is easy for warlocks to play around. 

I figured out why this card sucks and how it could be better.

Low cost demon cards are aids.

Make it so Demonfuse actually "Infuses minion with demonic energy."

Give a minion(any minion) +3/+3, and turn it into a Demon.

I.E. Turn 1, Argent Squire. Turn 2, 2 mana spell = 4/4 with Divine Shield and Demon Class synergy, your opponent has 3 mana to deal with it.

I'd play that.

Only way this card is playable....Give +4+4 or +4+4 / deal 4 dmg to a minon....Basically 2 Demonfires in one with the drawback.

The way I see it is that it works like Sacrificial Pact, so you could potentially buff Lord Jaraxxus to get a 6/8 weapon and +3 health. 

Evaluating the mana value of this card:

1) Blizzard values a card slot in your deck as 1 mana. e.g. a 4 mana card and a 2 mana card together should give the similar results as a single 7 mana card. This can be seen by e.g. comparing Arcane Intellect to Sprint (Sprint = 2* Arcane Intellect for 1 extra mana) 
2) This means gaining a mana crystal is valued at 3 mana. Check: Wild Growth cost = 3*1 - 1 = 2; Nourish cost = 3*2 - 1 = 5. 
3) So casting this spell grants your opponent 3 mana in value.
4) +X/+Y is generally worth (X+Y)/2 mana.
5) Demonfire & Demonheart seem to cost 1 mana extra for their versatility. Velen's Chosen costs 1 more for the +Spell Damage
6) This means the expected cost of just the +3/+3 with the Demon limitation should be a 2 mana spell card, as it is. With the mana crystal effect added, it should be FREE.

Extra thoughts:

* Having it turn a minion into a demon seems a better deal if you are running Mal'Ganis and/or Demonwrath or, if cast on an opponent minion to remove its original tribe (Mech, Murloc, Dragon,...) synergy, still seems costly, but adds higher level depth to the game.
* Alternative: "Give a demon +3/+3 and 'Your opponent has one more Mana Crystal' "
* I can only ever see 3 scenarios where I would cast this spell if I got it for free:
1) Turn your opponents 4-6 attack demon into a BGH target.
2) Opponent has a Wrathguard or similar effect on the board.
3) It's past turn 10 anyway and I am relying on my opponent not having BGH/hard removal.
In other words, when it's either combo this with something or pass my turn.

DF. Demon Fire is the same thing pretty much. Just asking for removal...

It is a late game card that doesn't fit a late game style

And you are undervaluing forcing your opponent to make a decision and be reactive. Much less on sub-optimal threats. Demonfire is a much more middle of the road card. 1 more toughness and 1 more attack, while maintaining the minion on the board, is an attractive option for a more aggressive deck. It forces your opponent to play a reactive game, very early, which is valuable. For a demon deck, this spell is rarely situational. For most mid-range/demonlocks 2 drops are Eggs, Jugglers, and creepers. If the meta changes towards control/strong late game decks, this card will find a very competitive place to keep Mid-Range Demon Zoo relevant unless better 2 drops/spells are to be had.

I get this feeling they tested it without the drawbback and it was too good in warlock without one

Warlock got screwed. Thats the deal, deal with it. Hopefully we have some better options to go along with this crap. 

If this card would make the target a demon instead. I would play it in a control warlock.

Demonfuse + Sacrificial Pact

In later turns, drawback means little anyway. Also a potential +3 burst if you have minions. Synergy with Mal'ganis. Situational, but good. If only Blizzard weren't smoking cracks, they could make an interesting card.