The first line of “Invictus” begins the poem in the midst of adversity, an opening that the following stanzas will imitate in some way. It begins with a dramatic—if somewhat conventional—metaphor, comparing suffering to night. But the speaker doesn’t stop there with this comparison. Going one step further, night becomes a substance that “covers” the speaker, like tar perhaps. By nesting a metaphor within a metaphor, the poem establishes that it's going to operate on a highly figurative level.
That doesn’t mean the poem won’t use more concrete imagery—far from it, as this line actively engages the body by depicting darkness as a substance that physically touches the speaker. As a quick biographical aside, Henley’s physical body was an important element of his public persona and in his personal life. When he was a child, he lost his left leg to tuberculosis of the bone. However, as an adult his physical robustness and zest for life inspired Robert Louis Stevenson’s character Long John Silver in Treasure Island. Furthermore, during the writing of “Invictus,” Henley was undergoing an experimental medical treatment for tuberculosis of the bone, and was in danger of losing his other leg. It's no wonder, then, that bodily suffering runs as an undercurrent in “Invictus.” However, we aren't meant to read such instances of physical harm literally, but rather take them as representing the kind of intense agonies that life can throw at us.
This first line is also dramatic in that it both establishes the poem’s meter and switches up that meter. In fact, this happens in the very first syllable. The poem uses iambic tetrameter, or an eight syllable line of four iambs, following a repeating pattern of unstressed-stressed syllables (da DUM). However, the very first foot of the poem is a trochee (stressed-unstressed; DA dum) followed by a return to regular iambic meter:
Out of | the night | that cov- | ers me,
This means that the poem begins on an authoritatively stressed syllable. We immediately become aware of the speaker’s willfulness (who won’t even be constrained by meter for one line!). We are also taken by the force of this “out,” which emerges from silence just as the speaker emerges from the night of suffering.
This metrical switch-up also creates a space of two unstressed syllables between “Out” and “night,” so that when the second stress does come with the word "night," it seems to hit extra hard. Thus we’re left with an opposition between “Out of” and “the night,” which mimics the opposition between the speaker’s willfulness and the constraining forces of suffering (literally constraining, if we think of Henley bedridden in the hospital).
Finally, the first line ends on the stressed word “me,” which will also become its first rhyming word. This places an emphasis on the first person that will continue throughout the poem.