NOTE: this video wouldn't embed, but the link to YouTube should work. Really, this half-minute kind of sums up the whole thing.
I guarantee you, this is the scariest, creepiest, most Godawful thing you've ever seen in a movie. As with so many horror classics (like Roman Polanski's Rosemary's Baby, which scared the giblets out of me), I watched this as a kid sleeping in the den on the pullout bed. This was a special treat and only allowed on non-school nights, which was great, because all the best creature-feature films were shown on Friday night.
I am sure I watched this with my older brother Arthur providing running commentary. We both burst into a sort of terrified "auuuuuugggghhhhhh" (not "ewwwwwwww": it hadn't been invented yet) at the "reveal", which only lasts a fraction of a second.
Mr. Sardonicus is probably William Castle's creepiest film. He was the master of horror schlock, and in this one he came on at the start and said the audience could choose between two endings.: "thumbs up" (Sardonicus lives!), or "thumbs down" (bye-bye, smiley face!) Only one ending was made, of course.
When we first meet Mr. Sardonicus he's living in the standard spooky old castle wearing an eerie-looking mask. I remember a sultry woman with a heaving bosom (hey, do you think I remember the whole thing after 46 years?), and a horrible scene in which some sort of Igor-esque servant (played by Oscar Homolka: with a name like that, what else could you be but an Igor?) carries a covered bowl into a room. The door closes and we hear Godawful sucking sounds.
Later on the story leaks out: Mr. Sardonicus use-da be a regular sorta guy, but his Dad won a lottery and was mistakenly buried with the money. Well, Mr. S. was thrifty and decided to dig the old man up. On seeing the decaying, reeking corpse crawling with maggots, his face spazzed into the "winning smile" you see here, and, just as Mama warned us, it froze that way.
He looks like a dead fox or something, just bloody sickening! You can have your Paranormal Activity 3, your endless Halloweens, even your parade of "Stephen King's. . . " (Carrie, Thinner, Pet Sematary, etc. etc. etc.) This nanosecond has to be one of the greatest, most disturbing moments in horror.
What I particularly like about this clip is that you can hear someone reacting in the background with an "auggghhhhhhhhhh." (Some things never change.) Watch it, yes - but don't watch it alone.