Arni Brokenbrow

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.

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.

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.