Delve is likely the most powerful mechanic in the entirety of Khan’s of Tarkir. Unfortunately, delve is also the least intuitive of any of the mechanics in Khans from a deck building perspective. Raid, outlast, prowess, morph, and ferocious all benefit from being in decks with laser like focuses on one central theme: attacking, +1/+1 counters, morph, and big creatures. At first glance, delve cards seem to belong in a deck that focuses entirely on filling up your own graveyard in order to cast delve spells for as cheaply as possible. However, every delve spell you cast eats away a huge chunk of graveyard necro-fuel and in doing so makes each subsequent delve spell much more expensive. Like unemployed business majors in a recession, delve spells fight each other for the same resources.

This leads me to a well known anecdote about the most efficient creature in Magic’s history: Tarmogoyf. When Tarmogoyf was first printed in Future Sight, players approached it as though it was a graveyard based build-around-me card. Deck builders flooded their Tarmogoyf decks with every card type, self mill, and odd synergies to try to turn Tarmogoyf into a 2 mana monster on turn 2 or 3. Then people found the secret to playing with Tarmogoyf: it is amazing in almost any situation because games of Magic always involve cards of multiple types going to the graveyard through normal gameplay. Tarmogoyf is extra powerful in decks with lots of removal, discard, and fetch lands; however, it is strong almost any time it’s played. Although Tarmogoyf seemed like a build-around-me card, it was really just a hyper-efficient creature that complements an ordinary removal/disruption strategy; delve will be at its best when used the same way.

Most ordinary midrange, tempo, and control decks can afford to play 2- 5 delve cards with the expectation that they will get to cast at least one for a big discount. Though delve cards suck in your opening hand, there are few cards you would rather draw on turn 4 or 5. 2-3 copies of a given delve spell sounds like a good fit in most cases. Playing 2 powerful cards in one turn can be backbreaking and all of the delve cards allow for those types of explosive plays. Turn 5 Mandrills + Polukranos sounds pretty darn hard to stop. You don’t have to worry too much about filling your graveyard to the brim since almost all of the delve cards are still plenty playable if you can only delve for 2 or 3. Now if you play some cards that put cards in your graveyard and add value, like Satyr Wayfinder or Nyx Weaver, then you can really go nuts with delve. I’m talking about playing 5-8 delve cards with a reasonable expectation that you will play most of them for 1-3 mana. I’m not sure if a dedicated delve deck will be as strong as just splashing delve into an otherwise solid deck, but it is certainly worth exploring. The delve cards that I would personally test in standard are Dig Through Time, Empty the Pits, Murderous Cut, Treasure Cruise, Necropolis Fiend, Become Immense, and Hooting Mandrills.

In limited formats delve works best as a splash, not a major theme. 40 card decks can really hurt themselves with self mill and running out of cards is a real threat. On top of that, most of the cards that self mill large amounts are not particularly efficient in and of themselves. For example, Rakshasa’s Secret has a strong enough effect that the mill is gravy, but Taigam’s Scheming doesn’t do a whole lot other than hopefully fix your draws and fill your dead guy pile. A better way to use delve would be to build a deck that plays for the long game and just happens to be able to delve for cheap near the end game. That kind of deck could conceivably play a delve card for its full cost or play it for almost nothing, which assures that your delve cards are never useless.

In short, delve shines in just about any deck with black, blue, or green and a medium to long game plan. If you have any doubts about the power of delve cards just look at this deck list that won a recent Legacy tournament. 4 copies of Treasure Cruise in the main deck of a Legacy tournament winner is about as big a statement as a card and a mechanic can make.