der Verfall
decay
The process of gradual deterioration or destruction, especially of buildings, morals, or strength.
Viele alte Schlösser in der Region sind vom Verfall bedroht.
Many old castles in the region are threatened by decay.

decay / expiration
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decay
The process of gradual deterioration or destruction, especially of buildings, morals, or strength.
Viele alte Schlösser in der Region sind vom Verfall bedroht.
Many old castles in the region are threatened by decay.

'Der Verfall' (decay / expiration) is the noun related to the verb 'verfallen' (to decay / expire). When something 'verfällt' (decays / expires), it is in the process of 'Verfall' (decay / expiration). The verb helps you understand the meaning of the noun as a process.
Ohne Pflege wird das alte Haus schnell verfallen.
Without care, the old house will quickly decay.
The meaning is clearly recognizable from context. If it is about buildings, strength, or morals, deterioration (m01) is meant. If it is about vouchers, tickets, deadlines, or claims, the loss of validity (m02) is meant.
Der Verfall der alten Fabrik ist nicht mehr aufzuhalten.
The decay of the old factory can no longer be stopped.
For the process of decay, there are fixed phrases such as 'vom Verfall bedroht sein' (to be threatened by decay) or 'dem Verfall preisgegeben sein' (to be left to decay). The area is also often named, e.g. 'der Verfall der Sitten' (the decline of morals) or 'der körperliche Verfall' (physical decline).
Das historische Gebäude ist vom Verfall bedroht.
The historic building is threatened by decay.
When referring to the loss of validity, the genitive or 'von' + dative is often used to say what is losing its validity: 'der Verfall des Gutscheins' (the expiration of the voucher) or 'der Verfall von Ansprüchen' (the expiration of claims).
Der Verfall der Garantie tritt nach zwei Jahren ein.
The expiration of the warranty takes effect after two years.
ascent / rise
'der Aufstieg' and 'der Verfall' can express opposites in a development. 'der Aufstieg' describes something becoming better, stronger, or more important. This can refer to a person, a company, or a power. 'der Verfall', on the other hand, describes something slowly becoming worse or falling apart, for example a building, an order, or someone's strength. The difference also lies in the direction of development: 'Aufstieg' shows an upward movement, while 'Verfall' shows a slow loss of condition or quality.

Der Verfall des alten Hauses war schon von der Straße aus zu sehen.
The decay of the old house could already be seen from the street.
Der Aufstieg des jungen Musikers begann mit einem kurzen Video im Internet.
The young musician's rise began with a short video on the internet.