Legacy has a long and storied past. Much of that past is mundane, as all histories are, and is lost in the subtle shift of metagames, the tweaks to known lists, and the failed brews that were never as good in practice as they seemed on paper. Once in a while though, a spark of brilliance shone through the mist, and we saw greatness emerge from the Aether. Often these decks were forced to burn out rather than see the format Fade Away, and so many players never got to experience their majesty. I have been fortunate enough to be there through them all, and have the opportunity to share some of what made these decks incredible with you all.

Without further pomp and circumstance, here is one man's list of the five most absurdly powerful, beyond-a-reasonable-doubt best decks ever to grace the Legacy scene.

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Cause – The power level errata on Flash is removed about a month before the GP. Prior to this change, the spell's effect only put the creature on the battlefield if you paid the additional mana – otherwise the creature went directly to the graveyard. Once this errata was removed, Flash became a two-mana, instant speed spell that afforded you the enters- and leaves-play triggers for any creature in your hand.
Effect – Flash is banned in March of 2007 after a single Grand Prix appearance and about two months of legality (as printed) total.

When you ask a long-time Legacy player what the most powerful Legacy deck of all time is, you will nearly universally find this deck as the response. For those who were there, this stands out as the most elegant and graceful Legacy deck of all time. It seemed like a puzzle whose pieces just naturally fell together in the most seamless way possible. When reminiscing about this list on Facebook, I stated "Body Snatcher exists as proof that God wanted this deck to win the Grand Prix." I don't know about any Divine Intervention, but I do know that the discovery of this deck and its associated technology were a heated topic in the weeks leading up to the event, and that this was by far the best deck I've ever played.

This combo* works through a complex loop to create infinite creatures. You Flash in Protean Hulk, and the leaves-play trigger finds Body Snatcher, Carrion Feeder, and sometimes a "protection creature" like Sylvan Safekeeper. You sacrifice the Body Snatcher to Reanimate Hulk, and sacrifice it to the Carrion Feeder. This iteration gets you a Karmic Guide, which again returns Hulk. You sacrifice again and get Kiki-Jiki. Now, you tap Kiki-Jiki targeting Karmic Guide, and with that ability on the stack you sacrifice Kiki-Jiki. This is important because it puts Kiki in the yard before its ability resolves, meaning the new copy of Karmic Guide can return him to play. You repeat this cycle about 6000 times, making an arbitrarily large number of Karmic Guides with haste.

*Legend has it that Steve Sadin, deep into day one of the event, was asked by an opponent to show the combo and he was forced to concede, as he didn't actually know how the combo worked. His opponents had all just conceded to the first Hulk trigger!

An interesting footnote in the discussion of this deck was the timing of that Grand Prix – it happened during the release weekend for Future Sight, and that set was not legal until Sunday of that weekend, meaning not legal for the main event. Once Future Sight did become legal, the deck quickly became closer to this list:

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I don't recall the specifics of this era of Flash, as most of the community was well aware the deck was heading for the banned list and didn't dedicate too much time to its development post-GP Columbus, but you can certainly see the extreme power in the deck. It became a pile of tutors for Flash and Hulk, alongside a miniscule kill package and massive amounts of protection. This deck was so fast and so powerful that the format devolved into a binary system of Flash and anti-Flash decks. The turn two kill was SO consistent that you really needed to play a Storm combo deck to be anywhere near its speed, and even then you were still more vulnerable to hate cards than this deck was.

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Cause – Unlike most of these decks, there was no direct printing or change to the format that acted as catalyst for this deck's existence. In no uncertain terms, Caleb broke it.
Effect – Survival is allowed to dominate throughout the remainder of the year. Eventually, with the printing of Necrotic Ooze in Scars of Mirrodin, the deck morphs away from the UG list to an Abzan shell utilizing Enlightened Tutor to find the Survival, and the Survival to find the Ooze combo. Survival of the Fittest was deemed too strong an engine and was banned in December of 2010.

The Ooze combo, for the unfamiliar, involved a Necrotic Ooze in play with both Triskelion and Phyrexian Devourer in the graveyard. You use the Devourer ability to continually put +1/+1 counters on the Ooze, and the Triskelion ability to Remove them, dealing damage to the opponent until they lose.

An Abzan Survival list with the Ooze combo:

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Gerry will be a recurring element in this time frame, as the early SCG Open series events were basically his sandbox – with everyone else just playing with the toys he left behind.

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Cause – In September of 2009, Entomb was taken off the Legacy banned list along with a few other tweaks. This was the first time since the creation of Legacy in 2004 that this spell was legal – it was included on the initial banned list largely due to the fear of Reanimator strategies.
Effect – Alongside Gerry Thompson demolishing the Open series with Reanimator, the deck began to show up online and in Grand Prix top tables as well. In a hotly debated move (seen as lacking proper justification by the vocal Legacy community) Mystical Tutor was banned in June of 2010.

Mystical Tutor was not seen as a big piece of the power-puzzle at the time of its banning. Many players contended that the real issue was Entomb, and as a spell with a history of banning it seemed to be the realistic choice. Wizards felt that between this deck - considered the obvious "best deck" at the time - and other decks using the spell like Storm combo or various other combo decks, the best choice was to Remove the powerful tutor from the format. The real debate surrounded their claim that Legacy involved an unspoken "gentleman's agreement" to not play the deck that was so obviously head and shoulders better than the rest of the decks in the format. Many players disagreed with this contention at the time; more factors than any such consensus were at play, but hindsight tells a different story. Half a decade removed from that era it does indeed appear the deck was much better than people were willing to admit, and those other factors at play were mostly stubborn adherence to an obsolete metagame more than any real objectivity. Time has shown that banning Mystical Tutor allowed Reanimator to be a strong contender but not the juggernaut it was in early 2010.

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CauseTreasure Cruise is printed in Khans of Tarkir, and as a result the entire format warps around it.
EffectTreasure Cruise is banned in January of 2015, and the format largely returns to pre-Cruise decks.

The most recent of the major powerhouses, UR Delver really exploited some of the parts of Legacy most would prefer swept under the rug and not spoken about. The combination of extremely undercosted threats, cheap card draw, and fetchland heavy manabases made putting seven cards in the graveyard a laughably simple task. Consider the mana curve of this deck: it quite literally caps at two for a resilient threat that overwhelms the opponent in rapid fashion.

Legacy tends to be self-regulating in most cases, and you often see decks fall in and out of favor as matchups begin to be prominent deterrents from degeneracy. When you see cards like Pyroblast and Forked Bolt showing up in the maindeck of Grand Prix Top 8 lists, you're looking at a deck that is far outside the homeostasis.

No one was surprised to see Treasure Cruise hit the banned list this January, and I think most Legacy enthusiasts were actually relieved. It's nice to step away from the monotony of traditional decks for a time, but those departures are better left as detours, rather than "the way."

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Cause – Dave Caplan and Lam Phan created a much more aggressive version of the top tier UGw Threshold deck that has been around since 2004. They promptly began crushing regional Legacy events across the northeastern US/Canada, and the world took note.
Effect – Dominant in Legacy for nearly the entirety of the format's history, adapting to changes within the card pool. The central strategy remained largely unchanged despite format shifts, as the Stifle / Wasteland package paired with efficient threats tends to invalidate your opponent's best laid plans.

No discussion of best decks would be complete without including UWR Threshold, Canadian Threshold, RUG Delver, or whatever your nomenclature of choice may be. Despite it never seeming to be the most objectively powerful deck in Legacy, it has - beyond all possibility of debate – seen the most consistent and long-lived success out of any archetype. From the Humble origins of Legacy to the highest heights of play, the UGr Tempo shell has been a mainstay. Nothing else comes close.

We can look to the very first major Legacy event for the origins of this strategy.

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The list may look odd, with lots of strange numbers – but remember that there was literally no metagame before this event. The decks played on this day were a strange amalgamation of things-not-banned-from-1.5, adaptations of old Extended decks, and random brews crafted in the local game stores across the US. Big Arse II, an event sponsored by early moderators of mtgTheSource.com was one of the largest DIY Eternal events to that date and set the stage for the first few years of Legacy. It was the event that players looking for an edge at Grand Prix Philly (the first Legacy Grand Prix, held in November 2005) looked to for decklists, because there just were no other options available. This was as wild as the Wild West gets, and really I'm surprised a deck like this one was even around back then. The basic structure of Werebear / Nimble Mongoose / Mystic Enforcer as threats, Brainstorm + the next best two cantrips, Daze / Stifle / Force lasted a very long time. For years after this event there was a massive line drawn in the sand between those who played UGw Thresh and those eschewing white for red. It wasn't until Caplan started destroying events left and right that consensus was achieved.

After a decade plus of experience with Legacy, I've seen powerful decks come and go. Despite the surge of adrenaline and fright that comes with a sudden and firm shake of the metagame, I find myself looking fondly on many of the Upheavals we see due to Temporary Insanity. Having opportunity to participate in these events makes them feel special and unique when looking back later on. Hopefully for those of you who were there as well, you enjoyed taking a stroll down memory lane with me. For those who didn't get the opportunity to share that experience, I hope you find these decks as interesting (and sometimes absurd) as we did in the moment.

Stay tuned, as I plan to return to this format shortly to deliver the five sweetest Legacy decks of all time. Where these decks erred on the side of strong, the sweet list errs on the side of awesome.

Adam
@AdamNightmare