The devouring force is God's own army, summoned by him, and what it announces is the day of the LORD, the day he comes to judge. Judgment runs creation backward, taking back a fruitfulness that was never owed to anyone.
Mercy hides inside the threat. A warning sent ahead of the disaster leaves room to turn around, and turning is what is asked: repentance, meaning the whole self swinging back toward God rather than a private flicker of regret. Fasting and ashes belong to that turning, the body agreeing with what the heart has decided.