I was 10 or 11 when I saw Mr. Sardonicus. I had nightmares for weeks (or longer).
Found a copy of it in my 40s, it seemed rather pedestrian.
It’s different watching horror as a kid and as an adult. When you’re a kid, you are very willing to suspend your disbelief and that’s what everyone behind the making of a scary movie hopes _everyone_ will do. Because while you can unsettle adults, they are always aware that what playing before them is not real.