Counter target spell. You can cast a non land card from your hand that shares a card type with the countered spell without paying its mana cost.
What a strange card. At first glance it looks like a severely over costed counterspell. In Legacy, I'm sure it could be used to cheat Progenitus or Emrakul, the Aeons Torn into play... so maybe it might have a use?
The most interesting thing to me is that with Counterlash, you can literally sit on a handful of counter magic and never tap your mana while you wait until you draw your win condition. With Counterlash, you never have to give your opponent the opportunity to cast anything if you don't want to.
Let's stick with Innistrad Block as see if we can make something of it. Let's make the most annoying Innistrad deck known to man. And to do that, we first start with a large compliment of available counter magic: