Eru Ilúvatar

Ilúvatar ("All-Father") is the Creator figure in Tolkien's works. He features most prominently in the early parts of The Silmarillion, where he created the Ainur (later many became the Valar and Maiar).

It should be noted that the Name of Ilúvatar, Eru ("The One"), is used by the Elves only on particularly sacred occasions. Using it casually in Middle-earth may lead to several reactions, none of them friendly.