I mean, that would make sense. Threndonites who were not around for the splintering do not turn into shades, I think. This means the splintering is the cause. Nazh also talks about proper rights and rituals to become a shade. If people automatically become shades when they die, then that must mean all of the people must become invested upon death, or they were already invested and something strange happens when they die. We can assume that the splintering caused a large amount of investiture to get thrown around, because the evil exits, the shades exist, and the deepest ones exist. Could some of the shades be animals? If humans turn into shades upon death, then why not animals? It would also explain why some shades seem more aware than others. Maybe the deepest ones are humans who went through the proper rituals, and kept all of their sentience, knowledge, and humanity.