Valmiki was a Rishi and the author of Ramayana. He was inspired to write the Ramayana by Narada and granted vision by Brahma. Lord Shiva previously gave him a boon that he will be famous because of his poem.
I become cleansed of all my sins. That dispeller of all sorrows, viz., the destroyer of the triple city of the Asuras, said unto me,—Your fame shall be great in the world.
- Mahabharat, Anushasan Parva, Section XVIII.