FOILCheapest priced copy (Kaldheim): $0.49 at Card Kingdom, near mint, their price list as of .
Arni Brokenbrow 
Legendary Creature — Human Berserker · 3/3 · Rare · Kaldheim Promos · #120s
Haste
Boast — {1}: You may have Arni's base power become 1 plus the greatest power among other creatures you control until end of turn. (Activate only if this creature attacked this turn and only once each turn.)
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $0.49 to $1.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Kaldheim | 120 | Rare | $0.49 | $0.49 |
| Kaldheim | 310 | Rare | $0.49 | $0.49 |
| Kaldheim Promos | 120p | Rare | $0.49 | $0.49 |
| Kaldheim Promos | 120s | Rare | — | $1.99 |
8 rulings from Wizards of the Coast
- 5 February 2021If an effect adds additional combat phases to a turn and a creature with a boast ability attacks more than once during that turn, its boast ability can still be activated only once.
- 5 February 2021Any effects that modify Arni Brokenbrow's power without setting it to a specific value will apply after Arni's base power is set, regardless of the order in which those effects were created. The same is true of counters that modify Arni's power.
- 5 February 2021If it's not your turn and you gain control of a creature with a boast ability after that creature attacked, you can activate that creature's boast ability if it hasn't been activated yet that turn.
- 5 February 2021A boast ability can be activated at any point after the creature with that ability has been declared as an attacker. This can be before blockers are declared, after blockers are declared but before combat damage is dealt, during combat after combat damage is dealt, during the postcombat main phase, during the end step, or, in some unusual cases, during the cleanup step.
- 5 February 2021In some unfortunate cases, the greatest power among other creatures you control may be negative. For example, if that greatest power is -3, you could change Arni's base power to -2 until end of turn. Arni strenuously objects to this course of action.
- 5 February 2021The greatest power among other creatures you control is determined as the boast ability resolves. You choose at that time whether you'd like to change Arni Brokenbrow's base power. If you control no other creatures at that time, you can change Arni's base power to 1 until end of turn. Arni objects to this course of action.
- 5 February 2021If a creature with a boast ability is put onto the battlefield attacking, it was never declared as an attacker. Its boast ability can't be activated that turn.
- 5 February 2021Arni Brokenbrow's new base power is set to the 1 plus the actual power of the other creature you control with the highest power, not that creature's base power. If that creature's power changes after the boast ability has resolved, Arni's power is unaffected.