Eru (the One), also called Ilúvatar (the Father of All or the Lord of All) is the name in the works of J. R. R. Tolkien for the supreme God, the creator of the angels (Ainur), the universe (Eä), the Earth (Arda), and its inhabitants. He is a single omnipotent creator, but delegates the day-to-day running of Arda to the Ainur. Eru is an important part of the stories of The Silmarillion but is not mentioned by name in Tolkien's most famous works, The Hobbit and The Lord of the Rings. (He is alluded to as "the One" in the part of LotR's Appendix A that speaks of the downfall of Númenor.)
The title the Father of All is clearly borrowed from the god Odin in norse mythology.
