Anticlimax (noun)

A disappointing or abrupt decline after a climax or high point.

Origin:

From the greek ἀντί (anti, "against") + κλῖμαξ (klimax, "ladder"), from the idea that an anticlimax is a "falling down" from a higher point of tension or expectation.

Examples:

1. The movie started off strong but quickly became an anticlimax.
2. The fireworks were a real anticlimax after the concert.
3. The party was going well until the cake turned out to be a anticlimax.
4. The proposal was so anticlimactic, I was expecting something grand but it was just a simple ring.
5. The end of the book was anticlimax as it ended abruptly and left readers disappointed.