This happened when I was eight and still visited (can't anymore because he did something that no child should ever go through to me )it was around fall I was round my dad's after he finally got to rent a room in a house after looking for a while. He got to get an extra room so he decided whenever me and my sister came over we could share it .
Time skip
My dad and sister were in his room and I got to the top of the stairs and just stared into our room and that when I saw it a tall slender creature wearing a suit . It was Slenderman and I staring at each other and then after a few seconds he was gone disappeared like it never happened.
