Wizards of the Coast is releasing Magic: The Gathering's new Kaldheim set soon, which is all about Norse mythology and heroic warriors in a frozen world. Already, a handful of previews have shown off some of the set's mechanical themes, such as Sagas and the emphasis on legendary cards, and the spoilers keep coming, such as Dragonkin Berserker and Battle Mammoth.
Every set has a mechanical identity, even if it's a top-bottom set (as in, the flavor and lore dictate how the cards work). Kaldheim is a land of heroic warriors and feats of strength, and the Boast and Foretell mechanics will help give this world some definition.
Dragonkin Berserker: Creating Dragon Tokens
Dragonkin Berserker is a 2/2 Human Berserker for {1}R, or a "bear." Since this creature is a rare, it's much stronger than regular bears, such as its first strike ability, but that's just the start. If Dragonkin Berserker was attacked in that turn and survived the ordeal, then it may pay {4}R to create a mighty 5/5 red Dragon creature token with flying.
Once in a while, Humans in the game have an affinity for dragons, being able to summon or morph into them, and the dragon planeswalker Sarkhan Vol would definitely approve of this berserker. It doesn't have to tap to make that Dragon token, but the reminder text makes it clear that the Boast ability may be used only once. What's more, Dragonkin Berserker makes all friendly Boast abilities cost {1} less for each dragon controlled, meaning it could potentially make a 5/5 token for just one red mana -- an incredible deal.
Overall, the Boast mechanic is reminiscent of Raid, which first appeared in the Khans of Tarkir set and reappeared in the Ixalan block. That ability rewarded players for attacking with creatures, and it could appear on creatures, instants and sorceries. It's clear that Boast will only appear on creatures, since it may only be activated if the creature attacked this turn, and instants and sorceries don't have activated abilities on them.
It's also likely that Boast will offer diverse abilities beyond creature token generation. Given this set's themes and previewed cards, other Boast abilities might deal damage to targets, create Treasure tokens, put +1/+1 counters on the Boast creature, create Elf Warrior tokens and more. Time will tell whether Boast appears in colors other than red. It's a strong possibility that Boast may be featured on at least a few green and/or white cards.
Battle Mammoth: Foretell the Future
The other previewed card is a green mythic rare creature: Battle Mammoth. As expected from its rarity and color, Battle Mammoth is a bone-crushing juggernaut, a 6/5 with trample for the low cost of {3}GG. That's generous, even for green mana, and Battle Mammoth isn't done yet. It can also cantrip when a friendly permanent (including itself) is targeted by an opponent's spells or abilities, which is a light punish for removal effects and the like.
Battle Mammoth also has the first confirmed instance of Foretell, which can only be cast from the hand. Anytime during a player's own turn, they may pay {2} and exile the Foretell creature face down. Starting on the next turn, they may cast it for its Foretell cost. It's not 100 percent clear whether every Foretell creature will have the cost of {2} to be exiled face down like that, but some mechanics have consistent costs.
In any case, this allows Battle Mammoth to be cast on turn four, in return for investing mana on turn two or three -- which means not casting other spells at the time. Players might be reminded of the Suspend mechanic from the Time Spiral block, though Foretell doesn't use time counters.
Foretell can be useful in more than one way. For one thing, it can help the player dodge hand removal effects, such as Thoughtseize or Despise. If the player Foretells their Battle Mammoth on turn two and then the opponent tries to use hand control on it, they're too late. Battle Mammoth is safe in its Foretell exile zone, and it may be cast later (though it can still be countered at that point, since it will use the stack as normal).
It's also not yet 100 percent clear how much of a discount Foretell costs may be, but it's very likely that each Foretell cost will be cheaper than the spell's actual mana cost. Battle Mammoth has a discount of {1} when using Foretell, and it's not impossible that other Foretell cards may have a discount of {2} or even {3}. Foretell costs might also reduce how much colored mana is needed to cast the spell, but as of now, it's uncertain if that will be the case. Alternate mana costs sometimes add or remove colored mana symbols from costs in this game, after all.