Final answer:
The poetic device used by John Donne in 'Holy Sonnet 10' when he writes 'Death, thou shalt die' is a paradox.
Step-by-step explanation:
When the poet of Holy Sonnet 10 closes his sonnet with "Death, thou shalt die," he is employing the poetic device known as paradox.
A paradox is a statement that contradicts itself but still seems true somehow. In the case of John Donne's sonnet, the paradox serves to challenge the conventional view of death as an all-powerful entity by suggesting that death itself can be overcome and will ultimately be defeated.
The use of a paradox creates a powerful conclusion to the sonnet, emphasizing the Christian belief in eternal life and the ultimate powerlessness of death in the face of salvation and resurrection.