This is my first experience I am sharing here; actually this is not my experience, but my cousin's.
We have a day in the Hindu calendar called "Kali Chaudas" (Dark Fouteenth). It proceeds the day of our new year. This is comparable to Halloween, minus the trick treat and many times darker.
There is a community called Tantriks (performers of black magic) in India, known to ward off ghosts, but pretty scary themselves. Well, these Kali Chaudas brought home to us the weirdest thing: I must have been about two years old or so. We were at my cousin's place for the New Year. My cousin sister is four years older than me. She was six at that time.
The Tantriks are known to set off wandering on this day. One of them was wandering outside my cousin's house. We are pretty used to this, so we didn't pay much attention. But he then came outside our door and stood there. We still paid any attention. My sister looked at him and suddenly screamed out loud. He then set off again. But pretty oddly, my sister went off scampering after him. All of us tried to stop her, but to no avail. It was as if she couldn't hear any thing. She just kept saying "I have to go." She seemed to possess so much power, that is was impossible for her to be controlled or stopped even by four fully grown people-her parents and mine. However, they managed to stop her and made her sit down. She continued saying "I have to go" she refused to eat or drink anything. She didn't even recognize us. She spoke nothing except that sentence.
That evening, the elders took her to a sadhu (priest). He told us that she was possessed or cursed. He told us to tie a necklace around her neck, place a sacred thread around her and make her drink holy water for a number of days. After drinking the water she recovered.
She still dreads kali chaudas and never meets eye with a tantrik... If any one knows about black magic, please tell me what this was...