OK SO THE LETTERS FROM PART TWO. I've seen a lot of speculation on them, and according to the Coppermind and general theory, the second was written by Autonomy (Or someone representing them) because Brandon has confirmed all the letters are from shards we know, and the third is obviously Sazed, the first probably Endowment. HOWEVER, I think the second letter instead comes from Trell. EVIDENCE: Trell's "faceless immortal" from BoM consistently referred to Trell or the general leaders the Set followed as "us" or "we." The second letter follows this pattern, though it does seem to be penned by something other than the shard, saying "you have spoken to one who cannot respond." This leads into my first theory about Trell. Trell is diametrically opposed to Autonomy, instead being the shard of unity or conformity, making everyone under its power the same, bound in a sort of hive mind. It explains the "faceless immortal," because Trell simply brought the beggar into their fold, making his mind a part of a larger whole. It explains why the one Hoid wrote to "cannot respond," as he wrote to the Shards original holder, but their consciousness has been blended with the minds of everyone under Trell's power. Furthermore, the letter also mentions "our many realms." This supports the idea that Trell is connected with the Scar (red band of stars across the sky in the Cosmere, connected because the threat to Scadrial is shown also as red smoke by Sazed.) The Scar is in reality all of the worlds and systems Trell has conquered. A looser connection, but it makes more sense when you think about it: wouldn't Trell, shard of conformity or unity, want everyone to be under his control, to be the same? Therefore, he/she/they would expand and expand, and now Scadrial is their obstacle. That conquest is also referenced in Obrodai, the world "we have claimed," suggesting a world that fell rather than succeeded. I immediately remembered that Brandon was/is planning a series where the main character is destined to destroy the world, and that seems pretty conveniently paralleled, as Obrodai sounds to be somewhere of great significance that Hoid is being warned away from. The main characters role in destruction there could be giving in to Trell. It all makes sense and fits together, if anyone notices flaws, please let me know so I can try to address them.