Three forms of figurative language that stand out in Thomas' poem are the contrasting light and dark imagery, the "good night," which is an example of both euphemism and irony, and the repetition of the word "rage."
Imagery is description that uses any of the five senses of sight, sound, taste, touch, and smell. In the poem, Thomas pits light imagery, connected with old men like his father fighting death, against the dark imagery of death itself. The narrator, speaking with passion, urges the elderly to "burn," "rage" like a fire, and "blaze like meteors" in the battle against mortality. Death, in contrast, is described as "dark" and as the "dying of the light." The light images are positive and appealing, implying power. The dark imagery suggests the loss of power in exchange for nothingness.
The repeated use of "good night" is a euphemism or polite way of saying "death." It is also an example of verbal irony because it means the opposite of what it says: death is not good, but bad, because it is dark, and darkness is a negative state of being in the poem that the elderly are urged to fight against with everything they've got.
Repetition adds emotional intensity and weight to a word or group of words, as well as a sense of rhythm. The repeated use of "rage, rage" conveys the deep feelings of the speaker. He is not at all calm or detached about death, but urgent and insistent in his demand that his father struggle fiercely to stay alive. Throughout the poem, Thomas uses figurative rather than literal language because it adds to the emotional impact of the poem's theme that the elderly should not accept death.