Dr. Jekyll y Mr. Hyde

Strange Case of Dr Jekyll and Mr Hyde is an 1886 novella written by the Scottish author Robert Louis Stevenson. It is about a London lawyer, Gabriel John Utterson, who investigates strange occurrences between his old friend, Dr. Henry Jekyll and the misanthropic Mr. Hyde. In a twist ending, it is revealed that Jekyll and Hyde were the same person, and that Jekyll had regularly transformed himself into Hyde by drinking a serum.

Synopsis from Wikipedia, CC BY-SA 4.0. Everything below written here.