You can find the first five parts of the series here:
Once upon a time, aggressive Legacy decks came in many forms. You might encounter someone playing Zoo, Burn, Threshold, Merfolk, or Goblins, or even a smattering of aggro decks based in white, black, or artifacts. Today, most of those strategies are endangered or completely extinct. They've all given way to strategies built around the same little blue creature.
Blue Delver decks are the best aggressive decks in Legacy for two basic reasons. The first, quite simply, is the card Delver of Secrets. At a time when Wild Nacatl was one of the best creatures in Magic, Insectile Aberration offered comparable stats and flying for a smaller deckbuilding cost. The second reason is that blue is generally the best color in Legacy, and specifically the best color for an aggressive shell. Let's examine why this is true.
Early on in this Legacy Guide, I covered some of the best and most defining cards in the Legacy format. Right at the top of the list were Brainstorm (along with its cousins: Ponder, Preordain, Gitaxian Probe, etc.), Daze, and Force of Will. If many of the best cards in the format are blue instants and sorceries anyway, then Delver of Secrets couldn't be a more natural fit. You can even pitch it to Force of Will!
Specifically, Brainstorm and the other cantrips allow the Delver decks to function as well-oiled machines. You keep your density of spells high for "blind flipping" Delver, or you can use Brainstorm and Ponder to stack an instant or sorcery on top of your library to guarantee that it transforms. The cantrips also allow you to build your deck with a low land count (some builds of Delver use as few as 14 sources of colored mana), because you can comfortably keep 1-land opening hands so long as they contain a Brainstorm or Ponder. Additionally, having control over the top of your library allows you to shuffle away extra lands when you draw too many, facilitating an extremely efficient deck filled with 1- and 2-mana spells that rarely runs out of gas.
Blue is also the best disruptive color in Legacy, with all-purpose permission spells that are good against a wide variety of strategies. In a format as wide open as Legacy, this is extremely valuable. Why build a disruptive deck by choosing one card that's good against strategy X, one card that's good against strategy Y, and one card that's good against strategy Z when you can simply play Daze and Force of Will, which are good against everything?
All things considered, the odds are stacked in favor of blue players in Legacy, and aggressive Delver decks are among the best and most direct ways to put the format's blue cards to use.
Although Delver of Secrets is a ubiquitous card, and although all Delver decks share a familiar shell, there's still plenty of variety within the category of "Delver." Let's look at a few of the most popular.
Patrick Tierney, 21st place at GP Louisville
To me, no deck encapsulates the spirit of Legacy like Temur Delver (sometimes known as R/U/G Delver or Canadian Threshold). This deck takes the concept of efficiency to an extreme with the lowest possible land count, and the highest possible concentration of cheap spells. Patrick Tierney's deck, featured above, contains a grand total of 4 cards that will cost him more than 1 mana to cast!
Temur Delver is designed to play the leanest game of Magic possible, and forces the opponent to play that game with them. Wasteland, Stifle, Lightning Bolt, and cheap permission spells allow Temur Delver to trade off resources at a lightning-fast pace until both players are whittled down to nearly nothing. From that point, the deck with the ability to operate on one land—and the ability to shuffle away excess lands via Brainstorm and Ponder—is almost always going to beat the deck with the more complex, ambitious game plan.
U/R Delver uses a similar shell, but a different game plan. Instead of trading resources, U/R Delver wants to throw its spells directly at the opponent's life total. Prowess creatures like Monastery Swiftspear combine perfectly with the blue cantrips to help you unload damage while also finding more burn spells.
A particular hallmark of U/R Delver is Price of Progress. It's one of the most punishing burn spells in the format, but few decks are well set up to make use of it.
By now, it's likely that I've made some Delver experts angry with me by repeatedly referring to this category of decks as "aggressive." The truth is that all Delver decks, including the Temur Delver and U/R Delver decks featured above, are capable of taking on a defensive posture and winning longer games. In that way, Delver decks are their own animal which cannot be entirely categorized as "aggressive," "controlling," or "midrange." It's part of what makes them so difficult to combat.
4-Color Delver leans further toward the control end of the spectrum. It requires more mana to function properly (still not very much, mind you), but is rewarded with powerful spells like True-Name Nemesis and Snapcaster Mage. It also has a versatile answer card in the form of Abrupt Decay, which is particularly important in a world of Counterbalance and Chalice of the Void.
Grixis Delver and 4-Color Delver are similar. Sometimes many turns will go by before you, as the opponent, can identify which deck you're up against. The notable difference is the absence of Abrupt Decay and Snapcaster Mage, which lessens Grixis Delver's reliance on green mana and make it a bit faster and lower to the ground.
Sultai Delver is often slower and less focused than the red Delver decks, which it trades for the higher quality of individual cards. Deathrite Shaman, Abrupt Decay, and now Fatal Push are among the most appealing cards in Legacy.
If you think of Sultai Delver as mostly a "good cards" type of deck, then it follows that there's a lot of room for customization. Some players will turn to low-mana-curve versions with Stifle and Spell Pierce, in the vein of Temur Delver. Others will prefer to reach up the curve for True-Name Nemesis or Leovold, Emissary of Trest. Still, others might prefer Snapcaster Mage or delve creatures like Gurmag Angler.
There are a virtually limitless number of ways to build a deck within the Delver shell. Mixing colors and crossing strategies is very possible. So if you're a Delver player, feel free to experiment with different ideas and make your deck your own. If you're preparing to face Delver decks in a Legacy tournament, then you should learn to expect the unexpected, since you'll never know when an opponent might surprise you with a Stifle, a Hymn to Tourach, or an unconventional sideboard card.
In Legacy, it sometimes seems that pure aggro is a relic of the distant past. The wide range of Delver strategies have supplanted the pure aggro decks, and have excelled due (in part) to their ability to play both offense and defense, and to win long games in addition to short ones. They are a pillar of the format, and you'll have to contend with them any time you want to make a deep run in a Legacy tournament.