Rework coming soon.

If it was immune to spells or battlecries and was half the price maybe, but as it is this is one of the worst card values out there. I mean destroy your tempo for an expensive sea giant? Even when you only consider his positive he should be a 9 mana cost, since other illusive minions don’t carry an over-cost for that ability (Faerie Dragon is 3/2 for 2, Sister is 3/5 for 3, Knight is 4/6 for 5), so this thing costing 10 is already too expensive. Then when you add such a game breaking negative effect? Nope.
 
I mean look at Felguard, his battlecry “Lose a mana crystal” gives you a one mana undercost considering his taunt, while this one stays at 10? Even if this was a 6 mana creature it would still be hard to justify playing it, since it dies to so many things (BGH, Cobra, AOE, Battecries…), as it stands its unplayable.

**Play Big game hunter. **
 
“My greetings”.

@Ledvi  
Just on it’s stats, it fails a basic cost/value test, since it has only 8 attack. This means that for it’s cost, you would be better off playing another minion. Normally, cards that fail the cost/value test make up for it with their effect, in this case their battlecry. However in this case, the card’s battlecry is a drawback, meaning that it should reduce the minion’s cost rather than contribute to it. That’s why you can find cards like Zombie Chow, which are relatively powerful, but have a cheap cost because their card text is a drawback. Another example would be almost any shaman card with overload on it. Not only do you pay the mana cost, but also suffer a drawback in exchange for something more powerful than the initial cost would normally warrant.  
In this case, that drawback is insanely harsh. Again, comparing it to Deathwing, if you wipe the board of minions and discard your hand by playing Deathwing, you are still able to potentially play any card you draw next (since Deathwing coast 10, and without an additional effect can only be played at max mana). This card, however, not only doesn’t clear the board of enemies, but it resets you back to the start of the game in effect, meaning you’ll only be able to follow it up with extremely cheap minions or spells. And that’s assuming you haven’t already used them all already. On top of that, since your opponent still has his board of minions (if any), and full mana, unless he is simply out of options, will easily be able to counter this card. Ironbark Protector, an 8 cost minion, is a perfect counter to this card.
 
The only way I can see this card being useful, is in the fact that it doesn’t wipe your board. Which means you would need some way to give it charge that doesn’t have to be played after this card, and your opponent is already on 8 health (plus however much damage your already played minions could possibly do that turn).
 
Also, consider that it’s a legendary card, which means it’s value should be at least equal, if not greater, than it’s cost.  
My suggestion would be to either greatly reduce it’s cost (something like 5 or 6 mana maybe), or change it’s effect. It’s inability to not be targeted by spells or hero powers seems nice, but that still leaves it open to attack by minions, or the effects of another minion’s battlecry or deathrattle. If anything, simply removing the part where it destroys your mana crystals would make this card ‘on curve’ for it’s cost, as it’s immunity to spells or hero powers would make up for it’s lower attack. Altering it so that it randomly destroys either one of your own or your opponent’s mana crystals each turn would still keep the ‘spirit’ of the card, while adding a bit of RNG that could work in your favor, or against you.

@Ledvi
 
No, there are a lot of legendary that have extensive utility.
 
Dr. Balance’s anti-AOE, Peggles draw, Cho’s spell denial, Trion’s taunt and sword pop.
 
I don’t think I could see any situation where this card is worth playing unless you’re out of options. And honestly, if you’re in a situation where this card would be useful you’re prob on the ropes and should concede.
 
If it fails, you’re fucked. Doomsayer, aoe clear+ something like lobber or bomber, CC spells like frost nova, etc. This isn’t even that scary honestly, in many situations I wouldn’t even bother locking it down. Just throw an Annoy-a-tron up and watch my opponent cry that their mana is gone and a 1/2 stopped their big scary mob.

@DaringDo42  
Let’s hope that there aren’t any secrets that could bring him back with 1 HP or replace him with the same mana cost. Dear god, imagine if I get Deathwing randomly.

@Zincy  
Isn’t that the main purpose of legendary cards in general?

@TheDeinonychus  
Well what did you expect from a being that claims to be the incarnation of power and a god who rules the Pit? It’s a devestating effect, sure, but with spell cards that focus on AOE, the only three classes that could take this guy out on the next turn would be the warlock since he has Dark Bargain, Huntard with its cheap ass spell card Deadly Shot, and Druid spell card Poison Seed at the cost of its own minion being affected by the spell card. Facing an opponent with this card is like stalling or stopping the inevitable. You don’t want this minion to attack period. Silence, freeze, and AOE cards can do the trick. That is if you don’t use them early..

@Ledvi
 
It would at least be useful as opposed to, you know, utterly fucking stupid.

Yep, the ol’ Frost Nova-Doomsayer combo will put this sucker down.

@Zincy  
Hahahaha..no. I’m afraid I can’t do that. That’s just a waste.

So, destroy all your mana for something that would die to big AOE clear + like a madder bomber or bomb-lobber?
 
I’ll pass.
 
Make it a 10/10 with charge, would maybe be a good finisher.

Considering this is actually an even worse negative battlecry than Deathwing, which is a 12/12, this is a horrendously underpowered card for 10 mana.