Crazy DR theory.
What if the reason Ralsei knows so much about the prophecy and how dark worlds work is the result of a shadow crystal that he's not aware he has, or is even embedded inside of his body? Those things tend to give similar knowledge to those that look into them like Jevil and Spamton did; but in Ralsei's case, instead of going insane, he's just super depressed and anxious woth all that forbidden knowledge.