Merfolk is one the decks in Modern that is best suited to take advantage of the power of Force of Negation. The free counterspell gives the fish a premium interactive spell, something that was sorely lacking in this archetype prior to Modern Horizons. The aggressive blue tribe still seeks to end the game extremely quickly and it never has any shortage of blue cards, so the requirements and card disadvantage of Force of Negation are hardly any obstacle. Although Force of Negation can only protect your creatures for free on your opponent's turn, it is at its best at disrupting the unfair combo decks that would typically have a faster kill than an aggressive deck.
Modern Mefolk with Force of Negation
4 Cursecatcher 1 Harbinger of the Tides 4 Lord of Atlantis 4 Master of the Pearl Trident 4 Merfolk Trickster 3 Merrow Reejerey 2 Mistcaller 4 Silvergill Adept 4 AEther Vial 4 Spreading Seas 2 Vapor Snag 4 Force of Negation 1 Cavern of Souls 4 Mutavault 1 Oboro, Palace in the Clouds 1 Minamo, School at Water's Edge 13 Island (335) 4 Ceremonious Rejection 4 Grafdigger's Cage 1 Phyrexian Revoker 2 Deprive 3 Master of Waves 1 Ravenous Trap
The 1-drops are now actually something of a decision point in this deck. Cursecatcher used to be the only serviceable 1-drop merfolk, but the Standard-legal merfolk actually have some merit. Benthic Biomancer is appearing in some lists over Cursecatcher, as the minor upside of being able to adapt and loot once may be a similar or higher power level than Cursecatcher's ability. Mistcaller is a metagame call that actually has quite a few applications in Modern, primarily against the Hogaak deck that is currently running rampant through Modern. It's possible that Merfolk could now be running an increased number of 1-drops in the maindeck, and copies of Mistcaller could supplement the sideboard.
Everyone always asks why these Legendary lands are in Merfolk lists, so we'll get that out of the way. The upsides are extremely minor but do exist. Firstly, the opportunity cost of playing exactly 1 copy of each of these lands is essentially free. Cards that disrupt nonbasic lands such as Blood Moon or Field of Ruin are either terrible against this deck or better suited at taking out Mutavault. The upside of playing non-Island lands is minor protection against out Choke of sideboards. Oboro's upside can come against discard effects such as Kolaghan's Command or Liliana of the Veil. It can also provide extra blue mana in an awkward draw that involves multiple Mutavaults. You can float blue mana from Oboro, use the colorless mana to return it to your hand, and use your land drop to tap Oboro again. The upside of Minamo is comparatively even more fringe, but hey, maybe someday someone will be able to kill a Phantasmal Image that has copied a Legendary creature or pop a Kira shield.
It's also possible to build Simic Merfolk, primarily to gain access to lords numbers 9 through 12 in the form of Merfolk Mistbinder. Kumena's Speaker adds to the confusion on which and how many 1-drops are worth running. We've also gained access to a premium new blue/green land in Modern Horizons. Speaking of which…
It's most likely correct to be running at least 2 copies of Watterlogged Grove even in the mono blue version of the deck, and probably closer to 4 copies. It's true that Merfolk often races against other aggressive decks, but in any Aether Vial draw the upside of being able to cash in any number of lands is huge game.
That's the deck! The blue menace is likely well-positioned any time Force of Negation is relevant against a high percentage of the top decks in the metagame. We'll be sure to update this post soon after recording a video through a Modern league on MTGO. Got any tech/suggestions/interesting builds? Let us know in the comments! And of course, if you enjoy our Modern content, be sure to check out that sidebar and subscribe to our Modern feed!