So I have wondered for a while what "The Evil" on Threnody is that ravages the Homeland and first banished the people of that world to the so-called "Forests of Hell". Even in Arcanum Unbounded, Khriss (who seems to know a LOT about the Cosmere and the Shards) states that very little is known of it by anyone. But then, after reading the other things mentioned in Arcanum Unbounded related to Threnody, a theory formed in my head that seems completely logical to me but is very likely a RAFO kind of thing to ask.
So here's the backstory: According to the essay written by Khriss/Sanderson in Arcanum Unbounded, Ambition inhabited the Threnodite System shortly after the Shattering, but was eventually found there by Odium, who was on his vendetta to become the most powerful being in the Cosmere. The two Shards clashed violently throughout the three Realms, and their epic battle resulted in the Splintering of Ambition and the death of its vessel. The entire system was affected by it, particularly on the planet Threnody, which has two main continents. The smaller one consists of the Forests of Hell, the normal people, and ghostly creatures called the "shades" while the larger one (the Homeland) is dominated by the Evil.
The theory: As seen in the form of the shades, Cognitive Shadows of people exist on Threnody due to the way their world works. What I wonder is if people come back as shades after death, why not a Shard of Adonalsium itself as well? My theory, to put it plain and simple, is that the Evil is the Cognitive Shadow, or shade, of Ambition itself. A Cognitive Shadow made from an entire Shard is not unprecedented, mind you. Take the Stormfather on Roshar, for example. After Odium Splintered the Shard Honor and killed its vessel as he had done to Ambition (and others), an enormously powerful Cognitive Shadow formed out of Honor in the form of its spren, the Stormfather. I believe that the Evil on Threnody is the same thing in regard to Ambition as the Stormfather is to Honor: a really big, really dangerous Cognitive Shadow. After all, what do the shades do on Threnody? They attack those who break the Simple Rules, devouring their souls and dissolving their bodies. Who's to say that the Evil, as a Cognitive Shadow of Ambition, isn't just a shade like all the others? It was a person who died, and on Threnody, those come back as the shades. But seeing as this one was so powerful (a storming Shard of Adonalsium), then wouldn't that make it an immensely powerful and really huge shade that can consume an entire continent and feast upon the souls of men in multitudes (as as it is described doing in Arcanum Unbounded)?
So there's my theory. The Evil is Ambition's shade. What do all of you think?