Looks busted form me. at 10 mana with warlock tap its 10/18 stats on board. You can trigger its effect even by Flame Imp to make it 13/20 in split bodies, or 2 flame imps for 16/22.
Opponent need to remove it from board before dealing damage to your face, and killing 4/12 isn't always that easy (one of the reasons classic Ysera is still playable).
Looks like a a buff to slow warlock if this archetype is yet to return (quite possible judging from released cards up to the date)
Edit: The more i think about this card the less excited I am. My first impression was WOAH, but when i tried to compare it with some of lately announced cards, it just looks less impressive. Perhaps this card is very strong, but not strong enough? It made me to change my mind from very good to playable.
Main issue with this card, it has no immediate impact, and we know that is something we want at turn 8+. I can see this card in slow value orientated warlock deck, if one should be playable. Perhaps it will be played, but rather as an addition to deck not core of it.
If the battlecry have "For the rest of the game" then it is awesome, actually can be a smart way to replace Lord Jarraxus, but it is not . This card is obviously a Control card but in any serious control matchup like vs Warrior or Priest they have zillion way of kill this dragon in a hearth beat.
Cinderstorm misses the dragon and all goes face .... 5 6/6 nether dragons lol and a 4/12. Or hunter hero power for 12 ok ill make a 6/6 you can do 2 for every 6/6 dragon you want to beat the snot out of you next turn. It looks fun.
it prevents your opponent from going facei if the game isnt over but against aggro you are mostly dead on 8 if you play this with a full board of 6/6 and against control it will just get removed so i dont see any warlock deck where you want to play this