I like the idea, however, the Silver trashing is really political for multiplayer games.
Also, you should add "from their hands" to the trashing clause.
About strength, I'll only analyze for 2 players.
Without the silver trashing, this is way stronger than Witch. The attack is in fact even stronger than Witch + Cutpurse, because if you do trash a Copper is equivalent, but if you don't, Werewolf attack is meaner. And -$1 for next hand is really harsh in slogs, which Werewolf itself seems to lead into.
It also snowballs: Once your deck is diluted with Curses and Coppers, it would be increasingly hard to have a Silver in hand to kill the Werewolf, so most likely you want to trash the Silver as soon as possible.
As a one-shot, its still incredibly mean. Have the other player gain a Copper and a Curse and trash a Silver? I think I would just load on Werewolves even more than I do with Mountebank or Witch. I'd probably take Werewolves over MB and Witch if they are both available. The only caveat is with Silver flooders, particularly trader. Those counter Werewolves pretty hard, but that is ok.
In sumary, I think this card leads to slogs more than the other 5-cost cursers, especially having +2 cards as bonus, because it does not add economy to compensate for your own Silver trashing. Maybe allow the opponent to trash Curses as penalty? Trashing anything? Trashing any non-Victory?
