Illuminatus is an entity. He is not very well known to most of us. He mostly speaks in dreams. He does not appear in them, just his voice booms with in the world of your dream. The question is why? Why not show his face?
Shadows are souls twisted from committing certain acts in life called sin. Shadows in pure form are black clouds of dust. However, they are not as powerless as they seem. Shadows tend to look for their vessels, or beings to possess. How they look for beings is still unknown. They could be anywhere, in fact, they could be your friends, telling you to do things you don't want to do.
Sin is a very specific act. What act is maybe considered a sin in the Judeo-Christiantity religion, could be different with in the religion of Buddhism. But the question is: What really is a Sin? It is mainly an act against nature. Greater sins could include Genocide, Political Assasination, Massacres, etc. While Lesser Sins include betrayal, a single murder, stealing, etc.