A sting in the tail
Definition 1 of 1
Pro Tip 1/3Story Endings
Used when a story or situation has an unexpected, often negative, conclusion that may change your perspective. 
I loved the novel until the last chapter had a sting in the tail that left me upset.
Pro Tip 2/3Plot Twists
Associated with sudden plot twists that surprise the audience, especially in films or books. 
The spy thriller was gripping, but the sting in the tail revealing the hero as the villain was astounding.
Pro Tip 3/3Unexpected Outcomes
Refers to an outcome that was not visible before and might be shocking or disappointing. 
He seemed kind, but his refusal to help in the crisis was the sting in the tail.