This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Tales from the Darkside: The Movie
The last segment, "Lover's Vow", focuses on an artist who witnesses a gargoyle kill a man in an alley. The gargoyle makes the man promise not to ever, EVER, tell anyone what he saw at the cost of his life. Apparently, making this promise proves more beneficial then he thought, as he immediately meets a beautiful woman whom he starts dating, and his artwork is bought by an exclusive gallery. Years later, he's happily married to the woman and they have two kids, but he feels guilty for keeping the secret about the gargoyle from her. So, he tells her... and it's revealed that SHE was the gargoyle the whole time, and that by breaking his promise she can't stay in human form. At the same time, their children turn into gargoyles as well. The man tearfully tells his wife that he truly loved her, as did she, right before she reluctantly kills him. It ends with the gargoyle and her children looking very solemn, before turning to stone.