Vilespine required a combo for the same effect, in most cases essentially requiring at least 6 mana to pull off, and people ran that all the time. This just costs 6 mana flat, has a better body, and can destroy any copies (including on the board.) It's better than Vilespine, which was a great card.
Really good tempo. Power plays usually happen at turn 6 (Reno the Relicologist, Kronx Dragonhoof, Emperor Thaurissan, etc.) This is a perfect answer to whatever your opponent plops down on 6. It gets stronger the longer games go, because it wipes out priority targets that require multiple damage sources to clear.
It opens plays, helps you push for lethal, and makes your opponent think twice about relying on multiple copies of taunt minions to avoid lethal. It's not even a Combo effect. Auto include, I'd say.
Does "wherever they are" include dead copies in the rez pool?
Perhaps, cause you could still buff c’thune after it died
Yeah but I think in this case it's a bit different because it "destroys" the targets. And where do destroyed minions go? To the res pool. There is no "exile" or "banish" zone in HS, like in MTG or Yu-Gi-Oh.
It is good even when it just destroys a single minion on the board. The Black Knight was played occasionally when you knew you were likely to encounter Taunt minions, and for one less health, you can destroy anything. So the card is at least "good" and the special effect is just the cherry. It might do nothing (because you destroyed a legendary) or it might destroy your opponent's entire strategy.
The only thing I'm not sure about is the cost. While 6 is definitely reasonable, it might be a little too expensive, especially if there are no high value targets that you are concerned with. Vilespine Slayer requires a combo, but it still costs 1 less. That's not insignificant, and Rogue decks usually aim for a lower mana curve.
Still, I think it's safe to say that this is one of the better legendaries Rogue ever got, and unless Rogue falls off entirely, it's probably a safe craft.