Keldon warriors don't need signals to tell them when to attack, but when to stop.

Trompetenstoß

Angreifende Kreaturen erhalten +2/+0 bis zum Ende des Zuges.

Trompetazo

Sonnerie de Trompe

Squillo di trombe

ラッパの一吹き

Soar da Trombeta

Trumpet Blast

Attacking creatures get +2/+0 until end of turn.

The sound of the trumpets lights a fire in the hearts of the bold and snuffs the courage of the cowardly.

Core Set 2019

"Do you hear that, Sarkhan? The glory of the horde! I made a legend from what you abandoned."
—Zurgo, khan of the Mardu

Battlebond

Keldon warriors don't need signals to tell them when to attack. They need signals to tell them when to stop.

Masters 25

Iconic Masters

Conspiracy: Take the Crown

An "attacking creature" is one that has been declared as an attacker this combat, or one that was put onto the battlefield attacking this combat. Unless that creature leaves combat, it continues to be an attacking creature through the end of combat step, even if the player it was attacking has left the game, or the planeswalker it was attacking has left combat.

Even though Trumpet Blast affects only attacking creatures, the bonus will remain for the rest of the turn.

Only creatures that are attacking as Trumpet Blast resolves will receive the bonus. In other words, casting it before you've declared attackers usually won't do anything.