A long one, so make yourself comfy...
A few years ago i worked as a nanny and one night, my employers and i 'played' with a ouija board.
I swear i didn't push the glass, and they are certain they didn't either, but a girls name was spelt out... G-R-E-T-A and an age of 6 years old, and then the dog jumped up and knocked the ouija board over, so we stopped playing it.
Didn't really give it any thought over the next couple of days.
The children had a playroom in the basement, and the daughter was playing in there one afternoon whilst i was pottering round, when i heard her talking to someone, i just assumed she was talking to herself. This went on for a few days, when i asked her who she was talking to, she said her friend, 'Rita'.
Over the next few weeks the dd became quite withdrawn and anxious. She suddenly decided she only wanted to go in the bath as the showers were bad.
Now it might be helpful to note at this point that we actually lived in Bergen, Germany, the town of the infamous Bergen Belsen concentration camp, and the house we were living in was literally a stones throw away from where the train tracks passed that would either carry prisoners to the camp, or that the prisoners would follow to get to the camp.
It all came to a head one day when i'd taken the dd swimming - you had to shower before getting into the pool, and the dd had a major meltdown when i took her into the shower cubicle, she started screaming' It's not water, it's not water!!!' over and over again, so i pressed the button and i swear there was a moment that stretched into a minute before the water came out - and with that the dd fainted.
Further questioning revealed that the dd's 'friend' was actually called Greta and was six years old, a little girl who wore her black hair in pigtails and had come to visit on a train....