§ Of the Rings of Power and the Third Age The Silmarillion

Yet there came at length a stay in the onslaught of Sauron upon the westlands. For, as is told in tile Akallabêth, he was challenged by the might of Númenor. So great was the power and splendour of the Númenóreans in the noontide of their realm that the servants of Sauron would not withstand them, and hoping to accomplish by cunning what he could not achieve by force, he left Middle-earth for a while and went to Númenor as a hostage of Tar-Calion the King. And there he abode, until at the last by his craft he had corrupted the hearts of most of that people, and set them at war with the Valar, and so compassed their ruin, as he had long desired. But that ruin was more terrible than Sauron had foreseen, for he had forgotten the might of the Lords of the West in their anger. The world was broken, and the land was swallowed up, and the seas rose over it, and Sauron himself went down into the abyss. But his spirit arose and fled back on a dark wind to Middle-earth, seeking a home. There he found that the power of Gil-galad had grown great in the years of his absence, and it was spread now over wide regions of the north and west, and had passed beyond the Misty Mountains and the Great River even to the borders of Greenwood the Great, and was drawing nigh to the strong places where once he had dwelt secure. Then Sauron withdrew to his fortress in the Black Land and meditated war.