Delve (Each card you exile from your graveyard while casting this spell pays for.)
, , Exile X cards from your graveyard: Target creature gets -X/-X until end of turn.
9/20/2014 The rules for delve have changed slightly since it was last in an expansion. Previously, delve reduced the cost to cast a spell. Under the current rules, you exile cards from your graveyard at the same time you pay the spell’s cost. Exiling a card this way is simply another way to pay that cost.
9/20/2014 Delve doesn’t change a spell’s mana cost or converted mana cost. For example, Dead Drop’s converted mana cost is 10 even if you exiled three cards to cast it.
9/20/2014 You can’t exile cards to pay for the colored mana requirements of a spell with delve.
9/20/2014 You can’t exile more cards than the generic mana requirement of a spell with delve. For example, you can’t exile more than nine cards from your graveyard to cast Dead Drop.
9/20/2014 Because delve isn’t an alternative cost, it can be used in conjunction with alternative costs.