Ah, the tale of the Yuki Onna...As long as you don't break your promise and talk about what you saw her do, you get to stay with her together, but...
Everytime I see it released in the form of a manga like this, I was always, always waiting for an ending where she just decides to just not care about the promise and decide to stay with him anyway...And I'm glad I finally got to see it.
The joy of consistent reproduction with a woman you have high chemistry with. Bravo Yuki-Onna. Bravo, nothing is more holier then a growing family and matrimony.
You know what I'm always wondering? In stories with supernatural/immortal girls the girls often say something along the lines of "You must stay with me forever" to their mortal lovers. Do they make their lovers immortal or do they mean those statements figurative? I kind of prefer the first option...
Yuki-onna are a "sort of ghost" in Japanese mythology. They apparently come from women who die in snowstorms, and basically lure travelers in the snow to their death by seduction. But also like most Japanese mythical creatures you can mate with it so... I wouldn't think of them so much like western ghost in that sense. Yeah, she did kill his master, and according to the original folk tale this is based on (you can find it if you google "Yuki-onna by Hearn" you'll find it and it goes into detail why she spared him (basically he was young and pretty.)