Why Mrs. Stapleton at dinner on the night Sir Henry was attacked?

You're likely referring to the character of Mrs. Stapleton from Arthur Conan Doyle's novel The Hound of the Baskervilles. However, Mrs. Stapleton was NOT at dinner on the night Sir Henry was attacked.

Here's why:

* She was at the scene of the attack: Mrs. Stapleton was the one who discovered Sir Henry's terrified state after the hound attack. She was in the moorland near the site, likely to lure him out or observe his reaction.

* Her presence was crucial to the plot: She was the one who provided the crucial information about the hound's attack, further fueling the mystery and suspense.

The person who was at dinner with Sir Henry and Dr. Mortimer on the night of the attack was Mr. Stapleton. He played the role of the charming and seemingly harmless neighbor, all the while hiding his true nature and involvement in the hound attack.

Remember, Mrs. Stapleton was a key character in the deception surrounding the Baskerville mystery, but her involvement wasn't about being at dinner with Sir Henry; it was about her presence at the scene of the attack and her role in manipulating the events that followed.