I don't know if it will help you find an answer but...Never.
You see,this one is a temporary casket/chest. You keep opening them,in the end you get this,but you can never open it,in another update,you lose this one and get another casket,when you open that casket. Bam! You find this again,just like playing with matryoshkas but instead of getting stuck with the smallest one,that smallest one gives a smaller every once in a while.
Hope it's enough to explain how this works.
I am handing out equal judgment to everyone at an affordable rate, call me Fix Price, When the king dies, make sure his grave is King-Size.
I mean this is just speculation but since the chests so far went up to level 10 and hero rank maybe they will be openable once lvl 20 and choosen by gods rank will come out and let you open a new chest for every few levels and ranks again?