Arni Brokenbrow — ruling 1

The 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.

Arni Brokenbrow — ruling 2

Any 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.

Arni Brokenbrow — ruling 3

Arni 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.

Arni Brokenbrow — ruling 4

In 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.

Arni Brokenbrow — ruling 5

A 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.

Arni Brokenbrow — ruling 6

If 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.

Arni Brokenbrow — ruling 7

If 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.

Arni Brokenbrow — ruling 8

If 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.

Arni Brokenbrow : règles, fonctionnement et interactions (MTG)

Arni Brokenbrow is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Haste Boast — : 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.)

How it works : Arni Brokenbrow

Original English text — Wizards of the Coast / Scryfall

#1February 5, 2021· Wizards of the Coast
The 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.
No other card is named in this ruling.
#2February 5, 2021· Wizards of the Coast
Any 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.
No other card is named in this ruling.
#3February 5, 2021· Wizards of the Coast
Arni 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.
No other card is named in this ruling.
#4February 5, 2021· Wizards of the Coast
In 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.
No other card is named in this ruling.
#5February 5, 2021· Wizards of the Coast
A 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.
No other card is named in this ruling.
#6February 5, 2021· Wizards of the Coast
If 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.
No other card is named in this ruling.
#7February 5, 2021· Wizards of the Coast
If 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.
No other card is named in this ruling.
#8February 5, 2021· Wizards of the Coast
If 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.
No other card is named in this ruling.