The last obstacle in the protagonist's journey/story.
Explanation:
Since there isn't much context to the story, I am assuming that "<em>The last rung on the ladder</em>" could either have a literal or figurative meaning. If it's a literal meaning, the character might actually be climbing a ladder and they're about to take the last step (the rung) to get to the top. However, it's more probable that the author means it in a figurative sense. I believe that "<em>The last rung</em>" is referring to a series of conflict in the protagonist's life, and the author refers to the ladder and its rungs as the conflicts.
<em>Protagonist's story</em> - the ladder (as a whole)
<em>Conflicts faced </em>- the rungs of the ladder (hence "<em>The last rung on the ladder</em>"
<span> to convey the horrors and barbarity faced by millions of slaves due to the oppressive laws of the state to criticize the white population for celebrating liberty while enforcing slavery on the black population </span>
