MH3 cards are Modern legal and are seeing significant play on MTGO. Now that people have had a chance to brew with the set, we’re seeing one deck start to dominate the online leagues: Chord Toolbox.
Chord Toolbox is a suite of decks that utilize Chord of Calling to find combo creatures to win the game. The most popular incarnation of the deck uses Yawgmoth, Thran Physician, and two creatures with Undying.

This combo allows you to create a recurring loop of dying creatures, and draw your deck. Assuming you have enough life, of course.
Now that Modern Horizons 3 is released, Chord Toolbox has won multiple back-to-back league events on MTGO utilizing the new cards.
There are two versions that have won recently. The first is a Bant build using Nadu, Winged Wisdom with Shuko and Outrider en-Kor to draw a massive amount of cards.

The combo works like so: Nadu, Winged Wisdom will draw you up to two cards per creature, per turn, when that creature is targeted by a spell or ability. If one of the cards you draw is a land, you put it onto the battlefield (untapped if able, I might add). With cards like Shuko, you can target your creatures with the equip ability for free to enable the card draw.
Also, it’s important to note that you’re not actually drawing the cards; instead, they are put directly into play or into your hand. This avoids abilities like Narset, Parter of Veils.
The insane part of this combo is that you can use the lands that come into play to play new creatures you draw. Then equip that new creature to Shuko twice and draw two more cards again. As you can see, it’s pretty easy to keep this synergy going until you draw your whole deck. It’s no wonder that Nadu is already one of the most expensive cards in MH3.
The other Chord Toolbox version that is putting up wins on MTGO is more the traditional Yawgmoth build; however, it is upgraded with Flare of Cultivation to increase the speed. Because the deck already runs multiple one-drop ramp creatures, Flare can be easily cast for free with the alternate cost.

Both of these decks have also been upgraded with 3-4 copies of Springheart Nantuko.
Springheart Nantuko especially goes into turbo mode with Nadu, Winged Wisdom in play. Those free lands you reveal and put into play will trigger its Landfall ability and create 1/1 Insects for free. And guess what we do with those 1/1 insects? That’s right, we equip Shuko to them and draw two more cards!
The full Bant decklist is below and also viewable here on MTGdecks.net, and the updated Yawgmoth build is available here.

Professional MTG players such as Piotr Głogowski are already singing praises for this Nadu build.
Chord of Calling decks aren’t the only ones shaking up the Modern format. MTG Rocks recently reported on a new Boros Blood Moon deck utilizing Phlage, Titan of Fire’s Fury!
MH3 has only been released on MTGO for less than a week, so it’s hard to say if these new decks will stick around and where the format will ultimately end up. So far, Modern Horizon sets have had a history of significantly changing the texture of the Modern format, so we will have to wait and see.
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