Its fun, but its terrible. Its for players who pray to RNGsus.
If it said “destroy a minion”, it would be OK at 7 mana, because its reusable (if slow). However, cards that destroy a targeted minion generally cost 4 mana these days, and often have other bonuses (Cannibalize), so being able to use it for free later in the game is essential if you want to run it, rather than the usual monster removal options. Having said that, having such a card on the board would mean your opponent would have to think hard before summoning anything big. TBH, it might even need to have its durability reduces to 2 in that case.
However, playing a 7 mana card that can whiff is bad. And playing one that can backfire is just terrible.
I guess Priest has a card that can undo the damage done by a backfire (Shard of the Naaru), but that essentially means using 2 cards and 8 mana just to the job Shard could do on its own.
The card needs a buff. Either:
1 - It only plays spells from the class of the user. If you play mage, you’re much more likely to get spells that inflict damage, you’re guaranteed not to buff the minion, and it becomes a reusable Rune of the Archmage, for 2 mana less. You get 4 spells instead of 20 mana worth, but that can be good or bad. If you play it in Priest, you’re taking a huge gamble. If you play it in Paladin… you play it on a friendly minion.
Of course, that isn’t very “Yogg Saron”. But playing a 7 mana card that can backfire often means you won’t have the mana left to clean up the mess it makes when it does. So…
2 - Just reduce the cost. A lot. Its completely overcosted at 7 mana. With its current randomness, 3 or 4 mana would be fine.
3 - Have the card “imprison” a minion by making it go dormant. Not very “Yogg Saron”, but definitely very “prison”.
4 - What the hell. Make the minion go dormant, and then do some random stuff.
Anyhoo… as for the achievement… words fail me. As the card is right now, how many games are you going to lose because of this card by the time you get 60 kills with it? Worst achievement ever.