A few days ago my friend, William and I were discussing causing illusions on each other. He tried to get me to see a light and I tried to get him to see rain, but we didn't tell each other what we were going cause the other to see. We both closed our eyes and let in the others energy. Afterwards William wanted to try again but I said that he will see snakes. Then he got mad because he's afraid of them, and I didn't know. Then when he went to sleep he dreamt of one killing him.
So my question is: did I cause it?
But I told William that it was probably his subconscious dealing with his fear.
The subconscious expresses itself through dreams and altered states. If he felt fear, which is rooted in the subconscious, it is perfectly understandable that he dreamt of them. Illusion is a valid practice, however, it does not work like a lot of new people think it does. It won't cause people to physically see something unless they are under drugs, to be honest. Illusion is more about directing away or attracting attention, most commonly. It uses subtle energy to create circumstances in one's favor or detriment. Examples of spirits that work heavily with illusion are the Fae (of the Celtic pantheon), Alfar, and Svartalfar (elves and dark elves of the Norse pantheon). I would suggest looking into them if you are wanting to look more into illusion.