Which latin phrase meaning 'great work', refers to the largest and best achievement of an author?

The right answer is magnum opus, in singular, and magna opera, in plural.

Explanation: Just to elaborate a little on the answer, it can be added that magnum opus literally means "great work," and it is an expression used to describe a literary piece, a musical composition, a painting, a film, or other example of artistic creativity and genius that stands as the best and most important achievement of its creator.    

the answer is slaves

douglas was a slave who turned into a statesman and he refused to celebrate the 4th of july until all slaves were emancipated or set free.

the eye wink at the hand; yet let that be / which the eye fears, when it is done, to see.