Perhaps his stats are a little bit off kilter, but I like his battlecry.

It stems from his ability in game to kill all the players almost instantaneously during his encounter! I justify his stats by having him vulnerable to non-battlecry removal. Probably OP.

Screw you tirion.

Hodir and Sons.

My entry is the Old God C'Thun who summons his loyal Silithids.

C'Thun summons 2 of the following 4 cards randomly at the end of your turn.

Hope you enjoy C'Thun and his set of Silithids. 

Neutral mind control with the twist of having to damage the minion with one of yours, since she costs 10 mana and you can't play anything else (unless you're a Warrior and Coin a Whirlwind).

Kargath Bladefist is one of the 4 boss encounters in the Highmaul Raid. He was the Warchief of the Fel Horde and the Chieftain of the Shattered Hand Clan. He was once a clanless slave orc to the ogres, and was promised freedom if he killed 100 orcs in arena combat. The ogre boss did not keep the promise, and locked him up underneath, to which Kargath reacted by cutting his bound hand, replacing it with a scythe, and leading a rebellion against the ogres. 

I wanted to see more minions and mechanics revolving around Warrior's armor hero power. This could seem overpowered, but we should keep in mind that armor can't be easily controlled as the enemy could easily whittle it down. As a drawback, he destroys your armor but gives it back to you once he dies.

Give a +1 if you like! 

Flavor text: He is the powerful ogre mage that rules the Gorian Empire and is located in Highmaul. SMOrc FACE SMOrc I'm ready, I'M READY TO GO FACE SMOrc Like or we will go face SMOrc Kappa

This is my submission. It's a powerful legendary but the effect is not easily controlled because a strategically important minion may be placed somewhere you can't kill it (good placement of your minions would be more valuable if this card existed), or even the number of minions may not match so that you kill nothing, and the body is killed by any 2 mana spell and most of 2 mana minions.

Edit: realised I didn't add warlock/demon to her just after uploading.

Before anything else, here is the basis for my entry which is Yor'sahj the Unsleeping.
http://wow.gamepedia.com/Yor%27sahj_the_Unsleeping

Art of Pheonix: Ramses Melendez@Infected by Art
Art of Pheonix Ashes: chevsy@Deviant Art 

It's not as OP as it initially looks. Imagine a Nerubian with deathrattle of summon a Nerubian Egg. Weaker stats compare to Nerubian and the Ashes is harder to be activated by opponent's AoE but has +2 health for each rebirth and 1 damage AoE upon rebirth.

Played on curve, turn 3, it's a slight tempo loss and can be killed like any other 3/3 drops. If the 0/3 Ashes gets killed 2 turns later, it's a free 3/5 at turn 5, which I think is balanced. As game gets longer, 3 attack gets less and less relevant.

Great vs aggro. Mediocre vs Control.

     RAID Throne_of_Thunder 

And nobody tried still to make Blackhand the Destroyer? Well, then I guess it will be my honor.

8 mana warrior legendary. 5/8. Battlecry: Equip a Black Hand (4/1 weapon, whenever you kill a minion with this weapon, it does'nt lose durability).

Play: Our bond is iron!

Attack: YOU WILL ALL DIE!

Flavor text: When nobody looks at him, he still cuddles with teddy bears while sucking his thumb.

Brackenspore, from the Karazhan raid.