Luke Skywalker learns that his nemesis (Darth Vader) is his true father. In which film does this revelation occur?

The following lines are said in Star Wars Episode V: The Empire Strikes Back.

• Luke Skywalker: I'll never join you!
• Darth Vader: If you only knew the power of The Dark Side! Obi-Wan never told you what happened to your father.
• Luke Skywalker: He told me enough! He told me you killed him!
• Darth Vader: No. I am your father.
• Luke Skywalker: No... that's not true! That's impossible!
• Darth Vader: Search your feelings. You know it to be true.

Here the parent-child relationship between two people is revealed. Luke thought that they were previously unrelated. But, they find out that blood, after all, is blood. This is a common idea noted in soap operas, plays, books, other films, etc.

Whether or not this revelation is a good or bad thing mainly depends on who is learning about the reveal. Sometimes an ally, a mentor or a non-romantic mysterious protector will turn out to be the parent of the person given the reveal, but other times, the reveal of who the parent is will be cause for serious anguish. This is the case for Luke in this instance. His parent (father) is someone who appears to be a bad type of person. He is a villain working for a very big and bad entity. Darth Vader (Luke's father) is the most infamous individual in each and every saga of Star Wars.