MagicPotion
Felix Felicis
Felix Felicis, called liquid luck, makes the drinker’s endeavors more likely to succeed for a limited time. Horace Slughorn offers a small vial as a potions prize. Harry wins it and later takes it to persuade Slughorn to give up the Horcrux memory. Hermione objects to using it in Quidditch or ordinary work.

First mention
Harry Potter and the Half-Blood PrinceChapter: Felix Felicis
Slughorn shows the prize in Potions and sets the brewing contest that Harry wins with the Prince’s notes.