Who is Sauron from The Silmarillion and what is their importance?

Sauron, also known as the Abhorred and the Dark Lord, is the greatest of Morgoth's servants. He is originally a Maia who serves Aule, but he chooses to follow Melkor in evil. The Sindar call him Gorthaur. He corrupts the Numenoreans and attempts to corrupt Elves and Men through the use of the One Ring. When regaining power, he disguises himself as Don Guldur, a sorcerer.