By tapping Diamond Valley you can sacrifice a creature in order to gain life equal to the sacrificed creature's toughness. Not only that, but its effect is able to be used after blockers have been declared.
While it's not really a card that can necessarily win you a game, it's a very fun card for EDH / Commander decks that are built around life-gain mechanics. It's pricey, but it's a fun card to play. You can use it quite a number of times in a game, so it's worth a look if you're willing to sink a few hundred bucks or more into acquiring a copy. , Understandably, this card is on the Reserve List, as not only is it powerful, but it would be very difficult to reprint this in today's Magic.
Have you ever played Diamond Valley?
~ Phoenix <3