It's dangerous to meditate on your own without a great enlightened Master, because there will be Maya's hindrance. Our souls are likely to get out during our meditation. Without the protection from the power of a great enlightened Master, the power of Maya, or lonely ghosts, Astral beings or devils of higher levels, will come and borrow our body. Our house will be occupied when we come back. If two or three persons are forced to live together, they may fight. That's why that kind of people will suffer a lot. We say he is "possessed". Being possessed is due to the uncleanness of our body, speech and mind. If we are virtuous, and meditate alone quietly, such terrible things will not happen to us. Therefore, a true Master will teach us the moral aspect first, at least teach us to keep the five precepts.
Many people got possessed because they don't recognize their own power of concentration or their teacher's power of concentration. If the teacher doesn't have that power, the student likewise won't have it, either. An enlightened master is different from normal teachers. He has already attained the Tao, received the Dharma and realized the power of concentration and Samadhi, so what he gives us is heart-to-heart transmission , and his heart has become great; he has become one with everything in the universe; he is omnipresent. He will come when you are meditating or dreaming. He will come and tell you something, teaching you or taking you to a higher level.