Death be not proud, though some have called thee
Mighty and dreadful, for thou art not so,
For those whom thou think'st thou dost overthrow,
Die not, poor death, nor yet canst thou kill me.
From rest and sleep, which but thy pictures be,
The adjective ‘dreadful’ used to mean awe or fear-inspiring, rather than today’s meaning of ‘awful’. So the two adjectives ‘mighty’ and ‘dreadful’ are similar in meaning, a device to give emphasis, known as  hendiadys . Another  example is Tennyson’s  Morte d'Arthur , where Excalibur is described as ‘mystic, wonderful’.
Death be not proud Holy Sonnet X