To unlock this prophecy you will have had to beaten Hades once. You will also have to had enough conversations with Nyx to unlock the the bonus perks that come with the Mirror of Night and you'll need to have used the keys to unlock all of the perks on the mirror as well. After you get your prophecy, you'll have to beat Hades using all of the perks available from the mirror. Technically this could be done in two runs, just pick your perks, beat Hades, then flip them all and do it again. Doing a run without Death Defiance is the hardest run to do, so for that one, I would have all of the perks that I like on besides that one. But to each their own.