* If referring to a gradual increase in volume or intensity: decrescendo or diminuendo
* If referring to a gradual increase in something else, like excitement or tension: decline, decrease, diminution, downturn, ebb, waning
For example:
* Music: The music built to a crescendo and then faded into a decrescendo.
* Storytelling: The tension in the story reached a crescendo during the climax, then began to wane in the resolution.