The ending was surprising and dramatic, I really liked the shift in power, and the shock felt by the two characters observing. It made me want to read more, and find out why the transformation occured, who was responsible and why they did it, and also what happens next. That is exactly what you want your readers to feel. Having said that, be careful of tenses in your narrative - you start out here with standard third person past tense (he said, she said)then switch to present tense (he sits, she stands). It is incorrect grammatically and also distracting. Either perspective/tense is acceptable, however the tense needs to remain constant unless you are effecting a deliberate change in point of view, such as switching the perspective from the eyes of one character to another. there are ways to do this, but they are specific and deliberate, and are used as a narrative device for a specific reason. Generally speaking you want to maintain agreeing tenses throughout. I thought the characters were interesting, and I found myself curious to know their history, and why Nathaniel reacts to Vivian the way that he does. Any time you can make a reader want to know more about your characters and story, it is a good thing. Is this part of a larger piece?