Consonance, as the name implies, is an effect where consonant sounds are repeated. While alliteration—the repetition of the same consonant at the start of a word—is a type of consonance, consonance also includes the repetition of internal consonant sounds. It should not be confused with assonance, which is the repetition of vowel sounds. In this poem, we can see consonance in the repetition of s in the line "Curse, bless, me now with your fierce tears," and also on t in the repeated line, "Do not go gentle into that good night." There are also multiple incidences of alliteration ("rage, rage," "sang the sun in flight," "blind eyes could blaze").
Allusion is when a writer uses an expression which recalls another work, a piece of Classical myth, or similar. There isn't an overt allusion in this poem, but the reference to "my father there on that sad height" is sometimes interpreted as a description of God looking into "the valley of the shadow of death," an allusion to Psalm 23.
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