Mark Z. Danielewski's The fifty year sword

On Halloween night, a recently divorced seamstress named Chintana attends a fiftieth birthday party set in a foster home that turns out to be for her ex-husband's mistress. A mysterious storyteller arrives to entertain the five orphans there. He keeps them spellbound with his tale of vengeance and his long black box that holds a magical weapon--a sword that cuts but shows no wound until the victim's fiftieth year of life. As midnight approaches, the box is opened, and the partygoers come face to face with the sword's deadly power.