verheerend
devastating
Having a catastrophic, destructive effect; resulting in very serious consequences.
Der Sturm hatte verheerende Auswirkungen auf die Küstenregion.
The storm had devastating effects on the coastal region.

devastating
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devastating
Having a catastrophic, destructive effect; resulting in very serious consequences.
Der Sturm hatte verheerende Auswirkungen auf die Küstenregion.
The storm had devastating effects on the coastal region.

The adjective 'verheerend' (devastating) derives from the verb 'verheeren' (to lay waste / to destroy), which means 'verwüsten' (to devastate) or 'zerstören' (to destroy). This destructive force is present in both meanings of the adjective.
Der Krieg verheerte das ganze Land.
The war devastated the entire country.
You can recognize the meaning from the noun: with concrete events such as 'Brand' (fire), 'Sturm' (storm), or 'Unfall' (accident), literal destruction is meant. With abstract terms such as 'Kritik' (criticism), 'Eindruck' (impression), or 'Ergebnis' (result), the figurative, disastrous meaning is meant.
Das Erdbeben hatte verheerende Folgen für die Stadt.
The earthquake had devastating consequences for the city.
In the literal meaning, 'verheerend' (devastating) is often combined with the nouns 'Folgen' (consequences), 'Auswirkungen' (effects), 'Schäden' (damage), or 'Verluste' (losses).
Die Dürre hatte verheerende Auswirkungen auf die Landwirtschaft.
The drought had devastating effects on agriculture.
In the figurative meaning, 'verheerend' (devastating) is often used with words that express an evaluation, such as 'Urteil' (judgment), 'Kritik' (criticism), 'Bilanz' (balance sheet / assessment), or 'Eindruck' (impression).
Die finanzielle Bilanz des Unternehmens für das letzte Quartal war verheerend.
The company's financial results for the last quarter were devastating.
harmless
'harmlos' and 'verheerend' are opposites when talking about consequences. 'harmlos' means that something causes no damage or hardly any damage. 'verheerend' means that something has very serious or destructive consequences. This can fit real damage, for example after a storm, but also figurative situations, such as a very bad decision. If the consequences remain small, you would be more likely to say 'harmlos'; if they are very serious, 'verheerend' fits.

Der Brand im Lager hatte verheerende Folgen für die kleine Firma.
The fire in the warehouse had devastating consequences for the small company.
Zum Glück war der Sturz harmlos, und das Kind konnte weiterlaufen.
Luckily, the fall was harmless, and the child was able to keep walking.