About three or four years ago, I was home alone as my parents were picking up dinner and my siblings were at college. I was on the ground level of our house on my laptop. I have my laptop set up in our living room so my back is to the wall. From here, I have a clear view of our kitchen, as we have a giant doorframe but no real door.
With my peripheral vision, I can see most of our drawing room and the bottom of the stairs. Again, there's a large doorframe but no door. If you go down the stairs and go to your right, you'll be out of my vision until you enter the kitchen, due to the wall of the drawing room. When I'm on my laptop, no one can go up or down the stairs without me seeing their movement. We also have a lamp in the drawing room by the bottom of the stairs, so I can usually fully make out a person and identify them without having to turn my head.
On this night, I was browsing different sites as one of my cats, Velvet, was curled up on the cushion next to me. I started to hear some loud noises upstairs, but ignored it. I assumed that it was our other cat, Shadow, or that I was hearing wind hitting the house.
I thought I heard someone step down a few stairs and pause, but I ignored it. I assumed that I was either imagining it or it was Shadow. My cats are both terrified of people other than our immediate family and hide if anyone is in the house due to the people who fostered them being abusive. Since Velvet was resting next to me, I assumed everything was fine.
Suddenly, out of the corner of my eye, I saw a black outline go down the last few steps of the stairs. Usually with the lamp on, I would be able to see colors, but I could only make out black. At the exact same moment that I saw this, Velvet jumped up, faced the stairs, and started hissing as her fur stood up on end. The only time I've ever seen her do this was once when I was younger and home alone. Some man kept knocking on the door and my cat stayed between me and the door, hissing, until I told her to cut it out. Upon hearing my voice, the man ran away. This man robbed a house five houses down from us that night, as he knocked multiple times and broke in when he realized no one was home. (He was caught, which is how we know he knocked to ensure no one was home.)
I quickly turned my head towards the stairs, but only caught a glimpse of the black figure turning to the right. They were out of my view. Velvet stayed next to me, hissing. She seemed to be following something with her eyes - something that was headed towards the kitchen. I could hear footsteps on the wooden floor as they approached. But, right when someone should have walked out into the kitchen, the footsteps stopped. Velvet stopped hissing and immediately settled down. After a few minutes of complete silence and me gathering up my bravery, I searched the hallway and the rest of the house. No one was there.
Since this incident, Velvet has never acted up like this again. I told my parents about it, but they were pretty dismissive of it.
The weird thing was that some items had been moved around upstairs. And not little items, big items, like desks and a shelf full of supplies. They were all moved a foot or so away from where they normally are, which explains some of the sounds I heard. But, no one was in the house.
Seeing that I was only able to see black when I should have seen some color (and the blackness was all completely smooth with no bumps, not like someone was wearing a black shirt with a black mask), the sudden disappearance of the person, my cats reaction and the objects moved upstairs, I'm certain I saw a ghost. Judging by my cats' reaction, it wasn't a good spirit either. I've yet to see anything like this again. I do often hear someone walking up and down the stairs, but I do believe that this is a different entity. The other footsteps I hear sound like someone just normally walking up and down the stairs, while these were hesitant, almost as if they were trying to not be heard.