The youngest brother, too, starts to waste away. This is particularly sad because he's the spitting image of his mother, and was always everyone's favorite.
And he's the speaker's last surviving brother, and the dearest of all of them! So the speaker tries to stay strong to help his baby brother – he can't bear the thought that his darling youngest brother will die in prison.
But even though the youngest brother's cheerful spirit never fails; he starts to waste away in prison.
The speaker says it's terrible, under any circumstances, to watch a person die – and he's seen plenty of people die.
He's watched people die violently ("in blood"), and he's seen them drown on "the breaking ocean," and he's seen people die of illness while "delirious" with fever.
Seeing people die has always been horrible, of course, but watching his youngest brother slowly waste away is a whole new level of sad.
His baby brother slowly "fade[s]," but never loses his sweet personality – he doesn't cry about the idea of dying, but only out of pity for "those he left behind."
At first, the healthy-looking color in the younger brother's cheeks seems to suggest that he'll survive, but that color fades as gradually and as surely as a "rainbow" that fades in the sky.
His eyes are so bright that the speaker imagines that they almost light up the dreariness of the dungeon.
The younger brother is so patient that he never once complains or "groan[s]" about dying in prison, even though he's so young.
He chats about the "better days" they had before they were thrown in prison, and pretends to be hopeful to keep the speaker happy.
The speaker knows that his brother is going to die, and is totally miserable about it.
The younger brother starts breathing more and more slowly and softly.
The speaker strains to hear his brother's breath, but can't hear anything. He calls out to his brother, afraid that he's died.
Even though there's nothing he can do, he can't suppress his "dread."
The speaker thinks he hears a sound from his brother, and in a sudden burst of almost super-human strength, he bursts his chain and rushes over to his brother.
But he doesn't find his brother – he realizes that he's the only living thing in the "black spot" of the dungeon.
His youngest brother, his last living family member and his dearest brother, has died.
He compares his brother to a "link" between himself and his family, which has now broken – ironic, considering that they have been chained up in a dungeon together, and that he breaks the links of his literal chains just as his brother dies.
One of the speaker's brothers is dead in his shallow grave and one of them is now dead on the floor of the dungeon.
He takes his dead brother's hand, and feels that his own hands are just as cold.
He hardly has the strength to move, but he's not about to die or anything.
The speaker is almost "frantic" with the thought that he's not about to die, since everyone he loves is dead.
The speaker wishes he could die, but the only thing left keeping him going is his faith, which forbids him from killing himself or even allowing himself to waste away.