If you cast Angel of the Dire Hour outside of combat, its triggered ability will trigger but have no effect.

If a spell or ability instructs you to put Angel of the Dire Hour onto the battlefield from your hand, that means you won’t have cast it, so its triggered ability won’t trigger.

Creatures that were declared as attackers or entered the battlefield attacking continue to be attacking creatures until the combat phase ends. It’s possible to exile all attacking creatures after they’ve dealt combat damage if you cast Angel of the Dire Hour during the combat damage step or end of combat step.