das Unheil
calamity
A very great misfortune, a disaster, or an event with serious consequences.
Der schwere Hagelsturm hat in der Landwirtschaft großes Unheil angerichtet.
The severe hailstorm caused great calamity in agriculture.

calamity / disaster
Loading your saved state.
calamity
A very great misfortune, a disaster, or an event with serious consequences.
Der schwere Hagelsturm hat in der Landwirtschaft großes Unheil angerichtet.
The severe hailstorm caused great calamity in agriculture.

The word consists of the negative prefix 'Un-' and the noun 'Heil' (good fortune/well-being). 'Unheil' (calamity/disaster) is therefore literally the opposite of good fortune and describes a great misfortune.
Viele Menschen fürchteten, dass der Krieg neues Unheil über das Land bringen würde.
Many people feared that the war would bring new calamity upon the country.
'Unheil' (calamity/disaster) is an uncountable noun and is used almost exclusively in the singular. You do not say 'vielen Unheilen', but rather 'viel Unheil' (much calamity/a lot of harm).
Die Flut hat viel Unheil verursacht.
The flood caused a lot of harm.
'Unheil' (calamity/disaster) is often combined with certain verbs that describe a negative action. Typical combinations are 'Unheil anrichten' (to cause harm), 'Unheil stiften' (to stir up trouble/cause harm), or 'Unheil bringen' (to bring calamity).
Mit seinen Lügen hat er nur Unheil gestiftet.
With his lies, he only caused harm.
catastrophe
'die Katastrophe' and 'das Unheil' can both mean a great misfortune with serious consequences. 'Katastrophe' is very common in everyday language and in the news, especially for accidents, natural events, or major damage. 'Unheil' sounds more serious, somewhat elevated, or more dramatic. It is often used when something bad is threatening to happen or has happened, without describing it in precise technical terms. For concrete events like an earthquake, 'Katastrophe' is usually more natural; 'Unheil' fits better when you want to emphasize the evil or terrible aspect of a situation.

Die Familie bemerkte das Unheil erst, als Rauch aus dem Keller kam.
The family only noticed the calamity when smoke came from the basement.
Das Erdbeben hat eine Katastrophe ausgelöst.
The earthquake triggered a catastrophe.