Solarserpent

Perhaps the Ire worked with or messed with the people of Threnody. Both the Ire and Threnody seem to have very strong investiture in the Cognitive realm. Perhaps they messed up the Investiture that was left over from Ambition. That Investiture may have been controlling/containing the transformation of people into shadows (cognitive shadows). Removing/Changing it may have caused the Evil to occur. Seems like a nice idea but lacks any proof. Just that for some reason the Ire were afraid of something coming from Threnody, had specific defenses against it. They also wanted to obtain a shard for protection or some strong reason.

I feel like there was a big hint when one of the Heralds shows up and is trying to kill anyone who is a surge-binder because he believes it will bring back the desolation. Perhaps the Heralds told the Knight Radiant that their bond is what is keeping the monsters around, and the best thing they can do is just go away. Or perhaps the Heralds decided to break the knight's some other way.