anticlimax

Short definition from Wiktionary

Noun

1. 1
   A failed or reverse climax, particularly
2. 2
   A failed or reverse climax: An unsatisfying resolution to a narrative, usually owing to a deus ex machina or similarly trivial resolution of the main conflict.
3. 3
   A failed or reverse climax: An abrupt descent (either deliberate or unintentional) from the dignity of the idea which the speaker or writer appeared to be aiming for.