Flamewake Phoenix

Flying, haste
This creature attacks each combat if able.
At the beginning of combat on your turn, if you control a creature with power 4 or greater, you may pay {R}. If you do, return this card from your graveyard to the battlefield.

Foundations

Foundations Promos

Fate Reforged

Flying, haste
Flamewake Phoenix attacks each turn if able.
Ferocious — At the beginning of combat on your turn, if you control a creature with power 4 or greater, you may pay {R}. If you do, return Flamewake Phoenix from your graveyard to the battlefield.

Fate Reforged Promos

Flamewake Phoenix's last ability will check at the start of your beginning of combat step to see if you control a creature with power 4 or greater. If you don't, the ability won't trigger at all. If it does trigger, the ability will check again as it tries to resolve. If you don't control a creature with power 4 or greater at that time, the ability won't resolve. You won't get a chance to pay {R}, and Flamewake Phoenix won't return to the battlefield.

If Flamewake Phoenix can't attack for any reason (such as being tapped), then it doesn't attack. If there's a cost associated with having it attack, you're not forced to pay that cost, so it doesn't have to attack in that case either.