A statement in which there is a sudden fall from the serious to the trivial -

Anti-climax/Bathos:
A statement in which there is a sudden fall from the serious to the trivial or from sublime to the ridiculous.

Climax:
The peak of importance in a play or in a story. It is the point at which the rise of action ends and fall of action begins.
Catastrophe:
Catastrophe is the dreadful consequence of the story of a tragedy. It is the final scene in which the action ends with the death of the hero and other characters.

Source: An ABC of English Literature - Dr M Mofizar Rahman