anticlimax

Definition 
In fiction or drama, a falling off in intensity and interest following a serious high point. The term has a pejorative connotation when used to indicate a flaw in a narrative or play, unless a writer is employing it for a humorous effect. An apparent anticlimax can be, on closer examination, a brilliant enhancement of the scene. An example is the Drunken Porter scene following the murder of Duncan in Macbeth, which begins with an inebriated gatekeeper responding to a loud knocking on the gate to admit the noblemen Macduff and Lennox to the castle. On the surface the scene's comic tone seems inappropriate at such a solemn moment, but, as analyzed by Thomas De Quincey in his famous essay "On the Knocking at the Gate in Macbeth" (1823), the scene manages to intensify the tragic tone by suggesting that the murder has transformed Macbeth's castle into hell, while at the same time suggesting a kind of redemption with the entrance of Macduff.