Eru Ilúvatar is the supreme deity in J.R.R. Tolkien's legendarium, particularly in The Silmarillion. He is the creator of all things and the source of existence for the universe of Arda. Eru, often referred to simply as 'Ilúvatar', embodies the concept of a singular, all-powerful god who initiated the creation of the world through his thoughts and the music of the Ainur. In Tolkien's mythology, Eru Ilúvatar is not only the creator but also the guiding force behind the unfolding of history. He granted free will to his creations, including the Elves and Men, allowing them to shape their own destinies. This theme of free will versus fate is central to many of Tolkien's works, highlighting the complexity of his universe.