This pivotal moment is described in Harry Potter and the Half-Blood Prince, where the young Snape, consumed by his obsession with the Dark Arts and his desire to impress the "pure-blood" Slytherin crowd, uses a racial slur against Lily, a witch born to Muggle parents. This act of cruelty and bigotry irrevocably breaks their friendship and foreshadows the tragedy that will befall them both.