What was his major accomplishment?

Well,it depends on "who" you are talking about.We cannot answer if we don't know "who" this "he" is.Ask yourself before putting the question.

"Who am I talking about?"

"Who is this "he" that i mention in this question?"

"Will anyone have to "correct" me when I ask this question?"

"Can this question be "understandable"?"

"Can and will my question be "answered"?"