So judging by the content of the letters, it sounds like maybe Frost and Hoid were both a part of the initial shattering, not just Hoid. I would say it was either just the two of them or all of the original 16 shard holders as well. More than likely all of them as Rayse apparently wanted to become what he is now. So maybe some sort of grand experiment or power grab that now Hoid feels remorse for and a responsibility to put back the way it was?
Just rambling, don't mind me.