In lines 8–13 of "The Pit and the Pendulum," describe the narrator’s state of consciousness and how it changes.

Allowing six lines for the introductory quatrain and its translation, lines 8–13 of Edgar Allan Poe's "The Pit and the Pendulum" encompass approximately this section of the story:

I felt that my senses were leaving me. The sentence—the dread sentence of death—was the last of distinct accentuation which reached my ears. After that, the sound of the inquisitorial voices seemed merged in one dreamy indeterminate hum. It conveyed to my soul the idea of revolution—perhaps from its association in fancy with the burr of a mill-wheel. This only for a brief period, for presently I heard no more.

From this passage, we can deduce that the narrator is fully conscious at first. Since he feels his senses leaving, he was aware of his mental state and surroundings initially. He hears his death sentence distinctly. However, whether from mental distress at hearing the pronouncement or from some sort of physical trauma or both, he fades into a state of semi-consciousness. He is not completely unconscious, for he has a vague perception of the voices of the inquisitors and associates the "dreamy indeterminate hum" of their speech with a mill wheel. From that point, he loses more consciousness without completely passing out, for he "heard no more," even though he continues to visualize the proceedings without properly interpreting what he sees.
In this passage, the narrator moves from being fully conscious to losing some consciousness (his ability to understand speech) to losing more consciousness (his ability to hear and to correctly interpret visual stimuli). In this section, however, he never fully loses consciousness but remains in a state of semi-consciousness.

Lines 8-13 will differ depending on how the text has been printed in a particular book that contains this Poe story.  I will have to analyze the first paragraph of the story to make sure that I get these lines.  
Physiologically, the change in the man's consciousness is an easy question.  He is conscious when the paragraph begins, and he is unconscious when the paragraph ends. 
However, I believe that the question is probably asking about the man's psychological state of mind. He starts off okay.  He's scared when the paragraph begins, but he's in control of his mind; however, that doesn't last long.  The text tells us that he felt his senses leaving him.  

I was sick—sick unto death with that long agony; and when they at length unbound me, and I was permitted to sit, I felt that my senses were leaving me.

His physical senses start blurring the lines of reality and unreality.  He clearly sees the judges, and he hears their verdict, but after that his mind starts playing tricks on him.  He sees the candles that are on the table, and he envisions them as angels. The angels then transform from beautiful signs of hope to "meaningless spectres" with flaming heads.  It's at this moment that he acknowledges to himself that death might be good and restful. 

And then there stole into my fancy, like a rich musical note, the thought of what sweet rest there must be in the grave.

The narrator's final thoughts convey his knowledge that he's losing consciousness.  That's why the figures in the room seem to disappear along with the light.