What type of character is Sauron example of

A flat character, since he is two-dimensional, and doesn't change throughout the book.

Sauron is the title character and main antagonist of J. R. R. Tolkien's The Lord of the Rings. In the same work, he is identified as the Necromancer, mentioned in Tolkien's earlier novel The Hobbit. In Tolkien's The Silmarillion, he is also described as the chief lieutenant of the first Dark Lord, Morgoth.