Final answer:
The niece in 'The Open Window' creates a fictional account of her relatives disappearing on a hunt, terrifying Mr. Nuttel during his visit.
Step-by-step explanation:
In Saki's short story "The Open Window", the niece fabricates a story about her relatives going hunting and never returning, leaving the window open in anticipation of their return. During this narrative, Mr. Nuttel was visiting in hopes of finding some solace for his nervous condition. The niece's vivid storytelling ends up terrifying Mr. Nuttel when he sees the supposed 'ghosts' of the hunters returning.