'That on the ashes of his youth doth lie', Which figure of speech has been used in the underlined phrase?

'Ashes of his youth' is a metaphor — youth is implicitly compared to a fire that has now burnt out, leaving only ashes. There is no 'like' or 'as' so it is not a simile. There is no human attribution to non-human entities (personification). There is no repeated initial consonant sound (alliteration). The implicit comparison of youth to a burnt fire/ashes makes it a metaphor.