der Untergang
downfall
The collapse or end of an era, an empire, a culture, or a power.
Historiker diskutieren die Gründe für den Untergang des Römischen Reiches.
Historians discuss the reasons for the downfall of the Roman Empire.

downfall / sinking / sunset
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downfall
The collapse or end of an era, an empire, a culture, or a power.
Historiker diskutieren die Gründe für den Untergang des Römischen Reiches.
Historians discuss the reasons for the downfall of the Roman Empire.

The word is made up of 'unter' (down) and 'Gang' (movement, process). So it literally describes a process of 'going down' or disappearing, which applies to all meanings.
Der Untergang des alten Systems war unvermeidlich.
The downfall of the old system was inevitable.
'Der Untergang' (the downfall/sinking/setting) is the nominalization of the separable verb 'untergehen' (to go down, sink, set). When something 'untergeht' (goes down/sinks/sets), it experiences its 'Untergang' (downfall/sinking/setting). This connection helps you understand the meaning of the noun.
Das Schiff ist gestern Nacht untergegangen.
The ship sank last night.
The noun 'Untergang' (downfall/sinking/setting) is very often used with a genitive attribute that specifies what is going down, sinking, or setting. The structure is: der Untergang + [genitive].
Der Film handelt vom Untergang einer ganzen Zivilisation.
The film is about the downfall of an entire civilization.
In historical or social contexts, 'Untergang' (downfall) is often used in fixed phrases such as 'dem Untergang geweiht sein' (to be doomed) or 'zum Untergang führen' (to lead to downfall).
Die Korruption führte zum Untergang der Regierung.
The corruption led to the downfall of the government.
For the setting of the sun, the compound word 'der Sonnenuntergang' (sunset) is used very frequently. It is a fixed and everyday vocabulary word.
Der Sonnenuntergang am Meer war wunderschön.
The sunset by the sea was beautiful.
end
'das Ende' and 'der Untergang' can both mean that something no longer continues to exist. But 'das Ende' is a general word and can be neutral, for example with a film, a course, or an event. 'der Untergang' sounds stronger and often more dramatic. It is used especially when an empire, a culture, a power, or an entire era collapses. If something simply comes to a normal end, 'Ende' is more natural. If something large and serious falls apart, 'Untergang' fits better.

Im Geschichtsunterricht sprechen wir über den Untergang des Römischen Reiches.
In history class, we talk about the downfall of the Roman Empire.
Nach zwei Stunden kam das Ende der Besprechung endlich näher.
After two hours, the end of the meeting was finally getting closer.