Historically, Merfolk was always a viable yet lower-tier Modern deck. In the past few months, it surged from 3% to 7% of winning decks, which is about the same level as RDW, Jund, and Tron, but still some 4-5% below Affinity and Twin.

Some would credit this surge to changes in the metagame. Merfolk has eight islandwalking lords and countermagic hosers in Aether Vial and Cavern of Souls, all of which are strong against blue decks with lots of permission. However, the removal in Modern is efficient enough that even many of those decks still have a reasonable Merfolk matchup.

Even when it's well positioned, the good matchups are loseable, and it's hard to tune and adjust weaknesses within mono-blue. Because the deck needs a critical mass of Merfolk, it has limited room for disruption. While Spell Pierce and Spreading Seas are all nice and efficient, they don't match up well against Tarmogoyf and Affinity.

And that's the real trick. Even if Merfolk tries splashing a color (I am guilty of this), the deck can't run enough of the right types of disruption to reliably deal with its problems. It just isn't in the cards.

Instead, what Merfolk really needs is a card that doesn't take up one of the precious spell slots. It needs a cheap, Vial-able body with the right creature type that can disrupt while applying pressure.


Harbinger of the Tides is exactly that. Imagine your Grixis opponent spending the first two turns cantripping to power out a fast Tasigur, and then you Vial in this bad boy to bounce it. With Harbinger, Merfolk got a printing that's capable of turning the tide in a losing race.

Merfolk didn't become the best deck in the format, but it's back in the conversation.

Merfolk

Paul Rietzl, Grand Prix Oklahoma City Top 8


Lands
11 x Island
4 x Mutavault
3 x Cavern of Souls
1 x Oboro, Palace in the Clouds
1 x Minamo, School at Water's Edge
Creatures
4 x Cursecatcher
4 x Merrow Reejerey
4 x Silvergill Adept
4 x Harbinger of the Tides
4 x Master of Waves
4 x Master of the Pearl Trident
4 x Lord of Atlantis
2 x Phantasmal Image
1 x Kira, Great Glass-Spinner

Noncreature Spells
1 x Vapor Snag
4 x Spreading Seas
4 x AEther Vial
Sideboard
2 x Vapor Snag
2 x Relic of Progenitus
2 x Spell Pierce
2 x Spellskite
2 x Remand
4 x Hurkyl's Recall
1 x Dismember
BUY THIS LIST
Because it runs many similar cards, Merfolk has a lot of redundancy to its draws, and the mulligan decisions are similar across most matchups. Merfolk tests format knowledge and metagaming less than most decks and instead rewards the pilot for mastering board states. It's unsurprising that Paul Reitzl Top 8'd a GP with it.

One of the perks of this deck is that it gets away with running quad maindeck Spreading Seas. Besides serving as a solid hoser for Tron, it also answers manlands, cantrips to smooth out draws, provides devotion for Master of Waves, and combines with the islandwalk lords.

The mana base has a few pseudo Islands in Oboro and Minamo as a reaction to Choke being a popular sideboard card, and I've seen some players run Wanderwine Hub for the same reason. In the end, a decent amount of the time they have Choke you'll have an active Vial and a developed board and it won't matter.

The reason this list doesn't play the full four Cavern (though that is an option) is because you still need some blue mana to cast spells and Phantasmal Image, and 13 blue sources is about as low as you want to go.

Some players shave various Merfolk to fit in some number of Dismember, Vapor Snag, Relic of Progenitus, or Spell Pierce, but I like that Paul's list is more threat-dense. The deck doesn't handle flooding well, and while low-impact tempo cards can be clutch in the right situation they can also exacerbate a threat-light draw.

Sideboarding

For the most part, sideboarding is intuitive. You can cut Spreading Seas against two-color blue decks, decks lacking utility lands, and mono-color lists. Harbinger of the Tides comes out against creature-light matchups like combo.
If the opponent isn't running counters, and you expect the matchup to be more attrition-based (say vs. Jund), then cutting Aether Vials is a fine choice.
Master of Waves is an army in a can, but it's not great when you're trying to race a combo deck like Storm, Goryo's Vengeance, or Amulet.
Relic of Progenitus comes in against delve decks, Tarmogoyf, and specific combo like Living End, Goryo's Vengeance, and Storm.
Besides Twin, Spellskite comes in vs. Bogles, Infect, Amulet (redirecting Slayers' Stronghold), and Lightning Bolt decks (including Burn). In this deck, it's a little worse than usual vs. Affinity and I wouldn't even bother. Stopping Arcbound Ravager from moving counters around isn't as good because the deck lacks removal to manually pop the Ravager.
Affinity is the worst matchup, and it's borderline unwinnable without a hate card. I strongly agree with Paul's full four Hurkyl's Recalls. Drawing one of them increases the chances of winning to positive, and does things that even Stony Silence and Creeping Corrosion can't do (killing Glimmervoid, answering animated lands). It's almost impossible to lose after casting a second.
Against Twin, you have four Spreading Seas and four Harbinger of the Tides to board out, which is why we have two Spellskites, two Vapor Snags, a Dismember, and some number of soft counters and Relics to bring in.
In the mirror, Lord of Atlantis gives the opposing team islandwalk. You probably want to board them out.
Other popular sideboard options include Hibernation, Tidebinder Mage, Chalice of the Void, and a slew of blue counters like Dispel and Unified Will.
Tips on playing Merfolk:

Merrow Reejerey untapping Vial is a tricky play that doesn't come up often, but it's potentially game-winning when it does. More often, you'll just be untapping lands to cast more Merfolk or activate Mutavault.
A lot of playing the deck is doing combat and lord math. Mutavault, Silvergill, and Harbinger will always be 1 power larger than the lords, and Cursecatcher will share the size of the lords.
Just like it did in Standard, Master of Waves pumps Mutavault.
If you Spreading Seas an animated land, it's still animated until end of turn. This could matter if your opponent leaves one back to block—you can't use Seas to remove it as a blocker that turn. That said, Todd Anderson once bluffed a guy on camera by simply enchanting the land and tapping his guys sideways.
If you Phantasmal Image a Mutavault, it'll copy the printed card (an untapped, unanimated land).
Tips on playing against Merfolk:

The lords don't give themselves islandwalk, which is why a Master of the Pearl Trident and a Silvergill Adept is less scary than double Master, as a blocker would hold back a single Master. Prioritize killing the lords and you'll have an easier time of it.
Just as it is against Zoo and Bogles, Engineered Explosives is one of the best sweepers possible.
Rending Volley is the best answer to a Kira, Great Glass-Spinner. Unfortunately, it's a narrow sideboard card in general, as most Twin players shave combo pieces post-board.
If the Merfolk player activates a Vial on 3, they could have a Kira or they could have a Merrow Reejerey (which you'll probably want to save your removal spell for). Each game will have its own risk-to-reward tension for responding to the Vial activation or waiting. Note that many lists run more than one Kira.
When playing Grixis, try and save your mono-black removal for Master of Waves (this is a reason to swap out a Terminate for Go for the Throat.) If you can't, Electrolyze + Snapcaster can deal with a lot of tokens, and blue and black creatures are fine for blocking the Master itself.