I Think that the last chest is in an undiscovered area behind Atheon there is a room behind him after you defeat him that all dark except for a crack in the back of the room with light pouring out of it as the grimore says this is from a guardian who was lost in the vault "I have made a wound in the vault. I have pierced It and let in the light. Bathe in it,and be cleansed look to it and understand" that's how I know the guardian pierced the vault. It also says "from my own light and from the thinking flesh of the vex I made a shield.the shield is your deliverance it will break the unbreakable. It will change your fate" this shield is the relic which I also Beleive has something to do with the secret room and possible chest. Turning all the red lights green in Athens chamber and the portal locations may unlock it as well. Comment what you think you have to do in order to get the nasty chest or add on to what I have. Together we can figure this out.