Delve

Delve
Keyword Ability
Type	Static
Introduced	Future Sight
Last used	Khans of Tarkir
Reminder Text	Delve (You may exile any number of cards from your graveyard as you cast this spell. It costs less to cast for each card exiled this way.)
Statistics	5 cards; 80% 20%
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	keyword:"delve ("

Delve is a cost-reduction keyword ability. It was introduced in Future Sight. At the time, it was only printed on three timeshifted cards. It was re-introduced as the clan specific mechanic for the Sultai Brood in Khans of Tarkir.

Rules

From the ()

Delve can't reduce the colored portion of a spell's cost.
When using delve, a player can remove a number of cards in his or her graveyard that exceeds the generic mana cost of the spell. Doing so will reduce the generic mana cost of the spell to 0, though the colored mana cost of the spell will still have to be paid.
While playing a spell with delve, you can sacrifice a permanent as part of a mana ability, then remove that card from your graveyard as part of delve. So a Blood Pet, for example, can be used twice to help play a delve spell. The same is true if the spell has an additional cost such as "sacrifice a permanent" or "discard a card."