Your sideboard consists of 15 cards which allow you to tune your strategy to adapt to match ups. But how do you choose the 15 cards that make up your sideboard? What cards are important? What strategies should you focus on targeting? How should you attack those strategies?

Building a strong sideboard is crucial for competitive play, as they let you adjust to your opponents decks which is especially critical against decks with strong linear game plans like Adramelech Black Magic. These decks focus on a specific type of effect, such as Eradication, and can be susceptible to various hate cards that target those effects. Without proper sideboard pieces to fight against these decks they can easily combo away without you having much interaction against them.

Your sideboard will largely be home to cards that target specific decks and archetypes which otherwise don't contribute significantly enough to your main game plan to be included in your main deck. Frequently you'll see sideboards focusing on cards like Concrete Catacombs. Catacombs on it's own isn't a beneficial card for any win condition, and doesn't provide any form of card or board advantage. The strength of Concrete Catacombs comes in how it can disrupt or outright shut down various strategies reliant on eradication effects. Another example is Provoke New Thoughts, which at the cost of a warrior and some consume locks your opponent out of discard effects for a turn, which can be especially relevant against decks like Onoskelis. These cards are known as "hate cards", since they "hate" out a certain strategy or synergy. These hate cards can swing match ups against decks that are all-in reliant on a certain strategy, and it's a good idea to pack some of these in your sideboard as they can be key in turning the tides in an otherwise difficult match up. 

Concrete Catacombs is one of the most relevant hate cards being played at the moment, with 6/8 decks in the top 8 of the recent Booster House Siege Series event running the card, and the other two decks being reliant on eradication effects themselves.

Majik Void Typhoon and Close The Gateway are also powerful hate pieces against eradication decks, whilst also being powerful against mill focused decks like Grimm. Notably, eradication focused decks like Castiel Lost Messiah Tempo and Gaia Unstoppable Control can run these hate pieces without halting their own strategies.

Peace of Mind is a hate-piece that prevents your opponent from discarding cards from your hand. This can be useful against Onoskelis, as well as any deck running warriors with Swipe, such as Fandorian Freedom Fighter.

Provoke New Thoughts as previously mentioned, is a powerful effect against decks reliant on discarding cards, mainly Onoskelis Fallen Control.

Ignite The Night is a one sided board wipe that sends your opponents warriors straight to discard, whilst turning off perish abilities. This can be a fantastic piece of utility for decks that can't easily get around Holy Counters or perish loops like Prysm, Soul-Collector Of The Grimm and Esmerelda, The Grimm Necromancer or two copies of Quake, The Unstoppable Terra Stone. Against Angel decks, Mortis Grimm decks, Alero Fandorian Midrange, or Gaia Unstoppable Control, this can be a great way to break through difficult to remove warriors.

In contrast, other cards in your sideboard may not be explicit silver bullets targeting certain decks, but rather cards allowing you to adjust your own game plan in response to your opponents. Cards like Dismantle, Deflect, and Mists From The Fata Morgana can be brought in or taken out to adjust how and when you fight against your opponents strategies. If your deck is reliant on eradication, you might consider bringing in more Dismantle effects to better combat fortified card like Close the Gateway and Concrete Catacombs.

Basically, rather than fight a specific synergy, you fight your opponent's overall strategy. Against aggressive decks you may not need to attack their specific mechanical synergies, but rather focus on cards that slow down your opponent, remove their threats, recover or protect your DCM, and cards that let you tax their aggressive plays.

If playing against controlling decks, maybe lean away from protective pieces like Mists From The Fata Morgana, and towards cards that allow you to break through their control tools. More ways to disrupt and push through their fortified line, cards that otherwise help to push in damage, or cards that help to maintain card advantage to avoid being drowned out in cards.

Not every deck is able to run Majik Void Collapse, but those that can should consider the card as it can be incredibly powerful into many match ups. Apart from Majik Void Collapse, there are a variety of other hate cards that can be ran to great effect in various decks. As previously discussed, Concrete Catacombs is one of the most powerful and versatile hate pieces currently available. If your deck isn't reliant on eradication effects, and eradication based decks such as Black Magic and Prometheus Arion are popular in your local meta, it can be worth the meta call to run a copy in your main deck.

This decision is of course heavily meta dependent, but can pay off well if your meta analysis works out.

Whilst sideboards are ultimately highly dependent on your specific deck and meta, there are a few options to consider to kickstart building your sideboard.

As you can likely guess from how much it's been mentioned, Concrete Catacombs is a fantastic card for any deck not reliant on eradication effects, and 2-3 copies can be an easy slot in for any current sideboards. If your deck runs it's own eradication synergies, then you can consider one-shot hate options like Magic Void Typhoon or Close The Gateway.

Mists From the Fata Morgana and Welcome to Alburdunn frequently show up in full main deck play sets. If your deck isn't running the full 5 however, consider filling the remaining copies in your sideboard for aggressive match ups. One of the newest campaign promos, Alburdunn's Mayoral Announcement is another fantastic mist against strategies reliant on effect damage like Castiel Lost Messiah Tempo and Adramelech Black Magic Combo.

The last universally powerful sideboard card I can recommend is another recent campaign promo: Galterius, the Wayfaring Knight. In any deck not reliant on a specific guardian synergy, Galterius is a fantastic card to bring in to accrue card advantage in games when your opponent is going first.

The other portion of your sideboard can vary wildly depending on what deck's you're planning to face, and how you're planning to beat them, but these cards can give you a base to start building and exploring off of.

Overall, sideboarding is a key component of competitive play, and even in casual play events it can be helpful to alleviate feel-bad moments against certain decks.