Final answer:
The literary device used in the mentioned line is personification, which attributes human qualities to non-human entities or objects.
Step-by-step explanation:
The literary device used in the mentioned line is personification. Personification is a figure of speech in which human attributes are given to non-human entities or objects. In this case, the Gentlewoman personifies the light by stating that it 'stood by' Lady Macbeth and that it is under her command. By attributing human qualities to the light, the Gentlewoman emphasizes Lady Macbeth's desire for heavenly absolution.