TL;DR: I was bum-rushed by a very large (~7 foot tall) deep black mass at an old apartment I lived in.
It frightened me more than half to death and made me question everything I once believed about ghosts (which was: "Ghosts don't exist!") It also made me question my sanity...until my husband saw it. The relief I felt that night was incredible.
Years later, after we moved out, I developed the courage to ask the company that managed the building if they had ever received any reports of hauntings or paranormal activity in that building and to my surprise they said, "Yes actually, we used to receive several reports. But we haven't lately."
I also did a lot of research on the history of the property, hoping something obvious would jump out at me. Nothing did but the property is in the Lakeview neighborhood of Chicago, which is quite old. So who knows what pieces to the puzzle I'm missing.