It's nothing 'spooky' or 'evil', and I'm sure it will all be explained one day, when science catches up with the reality.
I sat in a development circle at a spiritualist church in 2001, where I experienced my first mediumship. I can only describe it as someone dropping information into my head. I relayed it to the person I thought it was meant for, and they were stunned by it. Experienced mediums do this day-in-day-out at spiritualist churches all around the world, and they would be found out very quickly if they were fake or not up to the job.