1. （激动人心的事情之后）令人失望的情况，令人扫兴的事件
You can describe something as an anticlimax if it disappoints you because it happens after something that was very exciting, or because it is not as exciting as you expected.

1. a change from a serious subject to a disappointing one

Synonym: bathos

2. a disappointing decline after a previous rise

e.g. the anticlimax of a brilliant career