In there books on astral projection Robert Monroe and William Buhlman astral project from self hypnosis with out intending to. They did not see a demon on there first trip out tho. This may of been a dream , the demon could of been a thought form created by you out of fear , a real demon , or just a bad dream. There are many books on a.p. and lots of info on the net , here is a link
Astral Dynamics Down-Under Community desert rat