Here is a couple more ideas, Looking at the grimoire cards for Atheon, The Ageis, and The Mythoclast, The Atheon card states, "Just as Atheon side steps PAST and FUTURE"...is this referring to when we are fighting him and he is dancing in the middle between the gates? The Mythoclast card states, "perhaps it will reveal itself to you in TIME", referring to its purpose or other function. So during Time's Vengance maybe we can activate this function. The Aegis Relic card states, "I have made a wound in the Vault. I have pierced it and let in the light, bathe in and be cleansed, look to it and understand". Is the wound in the vault the relic, or maybe the tear were light comes in behind Atheon in the Vault? Or maybe both. Are we to look apon the relic for a key while it shields us? With these things in mind, i was thinking this.. While Time's Vengeance is active, could we all move to the back of the Vault, and while getting bathed, or protected by the Ageis, look through the Mythoclast and move through or out the back of the Vault. A fireteam would have enough time to reach the back before Atheon attacks if oracles were dropped fast enough. Or Once Time's Vengeance is active we "look to it" the Ageis, through the Mythoclast scope, perhaps needs both perfectly lined up on the axis(up, down, left and right and rotation) and it activates something in the mythoclast. Maybe shoot those 6 lights in the vault room that look like the mythos scope. But non the less somehow the relic activates the mythos hidden ability. Also the Ageis card says if you abandon your objective let the vault consume you, which may mean we shouldnt kill Atheon before doing something to get the chest, or just leaving Atheon completely in exchange for this chest, or a loop back to an earlier part in the vault which would be a casual loop that is mentioned several times, and having to work back to Atheon again.