Several epigraphs in certain Parts of The Stormlight Archive serve as a sequence of Letters as they reveal some hints at the history of the Cosmere and the Shards of Adonalsium, as well as at the history and personality of their authors. They set the stage for the conflict on Roshar: not only is this one world at stake, but the entire Cosmere might fall if the Shardholder of Odium, Rayse, gets what he wants.
The epigraphs are phrased as a letter, and are best read and understood all at once. For convenience in reading they are reproduced here.
The First Letter
Old friend, I hope this missive finds you well. Though, as you are now essentially immortal, I would guess that wellness on your part is something of a given. I realize that you are probably still angry. That is pleasant to know. Much as your perpetual health, I have come to rely upon your dissatisfaction with me. It is one of the cosmere's great constants, I should think.I do so now.”
Let me first assure you that the element is quite safe. I have found a good home for it. I protect its safety like I protect my own skin, you might say. You do not agree with my quest. I understand that, so much as it is possible to understand someone with whom I disagree so completely. Might I be quite frank? Before, you asked why I was so concerned. It is for the following reason: Ati was once a kind and generous man, and you saw what became of him. Rayse, on the other hand, was among the most loathsome, crafty, and dangerous individuals I had ever met. He holds the most frightening and terrible of all the Shards. Ponder on that for a time, you old reptile, and tell me if your insistence on nonintervention holds firm. Because I assure you, Rayse will not be similarly inhibited. One need only look at the aftermath of his brief visit to Sel to see proof of what I say. In case you have turned a blind eye to that disaster, know that Aona and Skai are both dead, and that which they held has been Splintered. Presumably to prevent anyone from rising up to challenge Rayse.
You have accused me of arrogance in my quest. You have accused me of perpetuating my grudge against Rayse and Bavadin. Both accusations are true. Neither point makes the things I have written to you untrue. I am being chased. Your friends of the Seventeenth Shard, I suspect. I believe they're still lost, following a false trail I left for them. They'll be happier that way. I doubt they have any inkling what to do with me should they actually catch me. If anything I have said makes a glimmer of sense to you, I trust that you'll call them off. Or maybe you could astound me and ask them to do something productive for once. For I have never been dedicated to a more important purpose, and the very pillars of the sky will shake with the results of our war here. I ask again. Support me. Do not stand aside and let disaster consume more lives. I've never begged you for something before, old friend.
—The First Letter
The Second Letter
The epigraphs to Part 4 of Words of Radiance form a reply to this letter. Again, they are best read at once.
I’ll address this letter to my “old friend,” as I have no idea what name you’re using currently. Have you given up on the gemstone, now that it is dead? And do you no longer hide behind the name of your old master? I am told that in your current incarnation you've taken a name that references what you presume to be one of your virtues. This is, I suspect, a little like a skunk naming itself for its stench.You, however, have never been a force for equilibrium. You tow chaos behind you like a corpse dragged by one leg through the snow. Please, hearken to my plea. Leave that place and join me in my oath of nonintervention. The cosmere itself may depend upon our restraint.”
Now, look what you've made me say. You've always been able to bring out the most extreme in me, old friend. And I do still name you a friend, for all that you weary me. Yes, I’m disappointed. Perpetually, as you put it. Is not the destruction we have wrought enough? The worlds you now tread bear the touch and design of Adonalsium. Our interference so far has brought nothing but pain. My path has been chosen very deliberately. Yes, I agree with everything you have said about Rayse, including the severe danger he presents. However, it seems to me that all things have been set up for a purpose, and if we—as infants—stumble through the workshop, we risk exacerbating, not preventing, a problem.
Rayse is captive. He cannot leave the system he now inhabits. His destructive potential is, therefore, inhibited. Whether this was Tanavast’s design or not, millennia have passed without Rayse taking the life of another of the sixteen. While I mourn for the great suffering Rayse has caused, I do not believe we could hope for a better outcome than this. He bears the weight of God’s own divine hatred, separated from the virtues that gave it context. He is what we made him to be, old friend. And that is what he, unfortunately, wished to become. I suspect that he is more a force than an individual now, despite your insistence to the contrary. That force is contained, and an equilibrium reached.
—The Second Letter
It is confirmed that we "may have read that Hoid has specific beef with Rayse." Given that the Letter is the only writing that mentions anyone specifically having a grudge against Rayse, this can be seen as an acknowledgement that Hoid wrote the Letter.
In Words of Radiance, a group of epigraphs seem to reply to this letter, indicating that the original writer had taken on a name that describes one of his "presumed virtues". This could be inferred to mean his "wit", which would serve to re-enforce the theory that Wit (known to be Hoid) was the author of the oiginal Letter. 
|Shards||Cultivation · Devotion · Dominion · Endowment · Honor · Odium · Preservation · Ruin · Bavadin's Shard · Survival Shard|
|Shardworlds||Ashyn · Braize · Nalthis · Roshar · Scadrial · Sel · Taldain · Threnody · Yolen|
|Terms||Adonalsium · Sliver · Splinter · Realmatic Theory · Intent · Focus · Investiture · Shardpool · Spiritweb|
|Other||Hoid · Seventeenth Shard · Chronology · Shattering · Worldhopping · Letters|