Eru (meaning the One or He that is alone in Quenya), also referred to as Ilúvatar (meaning the All High or the Father of All) is the supreme God of Arda and Middle-earth. He is the single creator, standing omnipotent, omniscient and benevolent above the Valar and Maiar. He has delegated most of his direct action on Eä to the Ainur, including the shaping of Arda itself.
In the Second Age, Eru buried King Ar-Pharazon and his Army when they landed at Aman in S.A. 3319. He caused Arda to take a round shape, drowned Numenor, and caused the Undying Lands to be taken "outside the spheres of Arda".
Eru intervened twice in the Third Age: when Gandalf died in the fight with Balrog in The Fellowship of the Ring, it was beyond the power of the Valar to resurrect him; lluvatar himself intervened to send Gandalf back, and when Gollum finally acquired the One Ring, it was by his intervention that made him trip and fall into the flames of Mount Doom.