That Terrible Halloween Night by James Stevenson, 1980.

It’s Halloween, and Louie and Mary Ann think that it would be funny to play a joke on their grandfather and scare him.  First, they try putting a scary mask on their dog, Leonard, but their grandfather just pats the dog on the head.  Then, Mary Ann sits on Louie’s shoulders, and the kids put on a big, old coat and a pumpkin head.  However, their grandfather still isn’t frightened.

When the kids ask him why he isn’t scared, their grandfather says that he doesn’t get scared much since “that terrible Halloween night.”  When the kids ask him what he means, he starts telling them about a Halloween when he was a kid.  He was out trick-or-treating when he saw a mysterious old house and couldn’t resist taking a look inside.

As the grandfather, as a kid, explores the house, he encounters all kinds of strange and frightening creatures.  (My favorite is the one that’s “the worst parts of a lot of things” just for the description.)

But, nothing in the house is as scary as whatever is behind the final door in the house, the one that the monsters warn him not to go through . . .

You never see what’s behind the door, but the result is the punchline of the story.  Typical grandfather way to frighten the kids!

One of the fun things about this story is that the grandchildren aren’t just listening to the story but are shown reacting to it as the grandfather tells the story, sometimes interrupting with questions or comments.  The pictures are drawn in a comic style, and much of the dialog is contained in speech bubbles in the pictures.

The book is currently available online through Internet Archive.