@Ledvi  
Nah, it really isn’t.  
Consider the competition this card has in the seemingly overpowered department. There are at least three cards at 10 that bring back 7 minions that died in the game for example - impossible to survive unless you have a huge minion advantage yourself, or a ton of Taunt, or a destroy-all card. All of which work on this card (and since it is only ONE card, only one Taunt or one “destroy random creature” (say, Deadly Shot) work on it too. Heck, I could list a ton of cards that deal with this instantly - consider how cheaply (2 mana) Freeze trap does it, for example.
 
Overpowered? No, not at all. You could say it is a bit weak at 10, in fact.

@Vree  
I’m actually reworking this card. This was back when I was an extremely salty and new to Hearthstone. Never liked the effect I gave. It’s both overpowered and gives unfair advantage.

Nice concept!  
I think it’s quite workable -most ‘standard’ decks that neglect (or can’t do) a healthy portion ofdraw-more cards do limit themselves to 1 card/turn eventually.  
You basically just have to climb back to 10 mana 1 turn at a time again, but by that point that really doesn’t matter much. It isn’t much different from other all-or-nothing cards like Deathwing, basically.  
I think it may be good and balanced.
 
Ironically, it is actually in the worst place perhaps in a Warlock deck, since they can overcome the card advantage through hero power…at the same time that only adds 1 card at a time, so in an otherwise full-chump deck, like the extremely popular Knife Juggler/Darkshire Councilman chump spam, it could be an interesting addition.

Goldshire Footman would be this card worst nightmare… :D

@Ledvi  
-It would fit the warlock theme, voidcaller would make it playable.
 
-Freeze AOEs, twisting nethers, vanish, dark bargain, sabotage, deadly shot, brawl, those are the spells that can completely negate this minion from the top of my head, I’m sure there are more. The when you consider the myriad of minions that can still take control/kill him…
 
-tartarus was also where guilty souls went to suffer. Really, it was hell in everything but name, so calling the entity a demon is not so much of a stretch.

@armadiloeater  
Warlocks are already suffering from self damage and minions with devastating effect against both players. We already have Felguard who takes away a mana crystal. I think warlocks had enough cards that does that.
 
Yeah, Tartarus can still be hit by aoe spells, but its going to just tickle him, unless its Dark Bargain where he can be removed from the game or Vanish.
 
Still you shouldn’t let the effect scare you though. And lastly, I really don’t see Tartarus as a demon, given with how Tartarus is a name of the Titan’s prison.

Well, now at least it’s reasonable value-wise when you only consider its positive effect, better than a regular illusive minion since it can’t be targeted by battlecries or effects. Still leaves it open for random spells or battlecries (AOE, Sylvanas, Mind Control Tech…) or simple effects (cobra, maexxna, freeze…), so if you only take the buffed up illusive aspect it’s good value, a reasonable legendary.
 
But again, when you take into account the drawback, losing all your tempo just to get one minion out, it’s just unplayable at this cost. Such a bad side should have a major mana undercost, as it stands it’s good value only if you forget about the “lose all your mana” effect. Even a druid would be hard pressed to come back from that tempo loss in end game.
 
Only thing I can think of that would make this minion playable is hoping to get it out without triggering the battlecry (alarm-o-bot, varian wrynn…). Maybe if it was a demon it would see some play, since it could be easily put into the battlefield thanks to voidcaller though. All in all, interesting idea, but still too costly considering the risk.

@Darth Sonic  
Well that’s what you get when you summon the ruler of the Pit (Underworld). Do you honestly think you’ll be above the consequence?

@Ledvi  
… Then this card is absolutely worthless. You can’t do anything once it dies.

@Darth Sonic  
No you don’t.

… That is an incredibly powerful effect, but I can’t decide if it’s worth the pay-off.
 
Do you get your Crystals back once it dies?