I was sitting on the floor of my basement watching TV when I happened to glance into a mirror (beside the TV) opposite the large window behind me that looked out into my backyard. There, I saw a large head with two huge, angular black eyes. When I looked behind me, I saw nothing outside, but I could see a reflection of the face in the window when I looked at it through the mirror. At first, I thought it was a trick of the light, but when it tilted its head slightly, I knew it wasn't caused by anything in my room, as I had the TV turned off now and I was alone. I was so terrified, all I could do was sit and stare at it through the mirror and back into the window behind me. I could see that it blinked several times as I watched it. After about 30 seconds of having a fight or flight response of just freezing, I finally worked up the courage to bolt out of the room and run upstairs, where my family was already asleep. Since I was too scared to stick around long enough (I don't think I've ever been that scared in my life), I never actually saw it leave.I'm sitting in the same place where I saw it a couple nights ago (nothing has changed position in the basement room like lights or anything) and I see absolutely nothing in the corner of my window. I took out a measuring tape the next morning and measured to where I remembered its head being, and found that it was about 4 feet tall, maybe a little shorter.Normally, the curtains to the window are closed completely, and when I'd be down in the basement by myself with the curtains closed, I would hear what sounded like movement outside. Oddly, our motion sensor light didn't come on that night I saw it, though, and the sensor is only a couple feet away from where I saw the entity. For the past several weeks, with the curtains closed, I would be in the basement late at night and alone, watching Netflix or TV. However, I would hear strange noises outside like movement, or even tapping on the window. Until I saw this entity, I never really took notice or thought these sounds were odd or out of place, as we often have deer, raccoons, rabbits, outdoor cats, and other animals in our backyard. I always assumed the noises were just animals, but now I hesitate to enter the basement at night now that we took the curtains down entirely to replace them.