In Harry Potter and the Half-Blood Prince, Harry wins Felix Felicis in Professor Slughorn’s first potion class.
Slughorn gives the class a task involving a Draught of Living Death. The student who produces the best result receives a small bottle of Felix Felicis, known as Liquid Luck. Harry succeeds because an old Potions textbook contains unusually helpful handwritten instructions, including corrections and shortcuts attributed to the mysterious Half-Blood Prince.
The prize later becomes important when Harry needs Professor Slughorn to reveal a memory about Tom Riddle. Harry pretends to take the potion, but Slughorn’s confidence that Harry has used it persuades him to act. The scene shows how the potion’s reputation can influence people even when it has not actually been consumed.
Felix Felicis does not guarantee unlimited success or replace skill. The film presents it as a temporary aid that makes fortunate outcomes more likely. Amortentia is another potion shown in the film, but it is a powerful love potion rather than the class prize.