Step 1: The error in the sentence lies in the word "obscure." "Obscure" means something that is not easily understood or is vague. However, illusions by their nature are already vague or unclear.
Step 2: In this context, the sentence is redundant. "Illusions" already conveys the idea of something unclear, so the word "obscure" is unnecessary. The sentence would make more sense if the word "obscure" were omitted.
Step 3: Therefore, the error is in the word "obscure."