Mightiest of the mighty,
Master of disharmony.
A name ancient in time and fit,
Not tainted by elvish rhyme and wit.
Melkor
Morgoth, also known as Melkor, was the greatest of the Ainur. He fell from glory when he disrupted the Music of the Ainur and defied the will of Eru Ilúvatar.
NOTE: This is another riddle that kills Free Peoples characters, even if you give the correct answer.
Tolkien Gateway: Morgoth