This is the whole poem.... so I guess you'll find the answer here... OUT of the night that covers me, Black as the Pit from pole to pole, I thank whatever gods may be For my...
Because he says he is unconquerable and no one can take control of him. (He was a slave)
If your referring to the poet who wrote "Invictus" his name is William Ernest Henley
William Ernest Henley was born on August 23, 1849.
William Ernest Henley (1849-1903), the poet