It's likely that the narcissists eyes become dilated due to being in a very heightened state of excitement or arousal--remember that they need drama and chaos in order to feel good about themselves. The conflict excites them; you are seeing a large pupil.
Narcissists also may have drugs or alcohol use as part of their malfunction, which might effect how their eyes appear.
During some states of conflict, the narcissist may “rage out" to the point of dissociation, which may cause their facial expressions and eyes to look unfamiliar, as the person you know has temporarily “checked out “ leaving a core of rage and fury to spew out hatred at whatever is threatening their fragile inner self.
Narcissists are not supernatural creatures, they are broken human beings with severe deficiencies in their brain function and emotional capacity. They do not have any unnatural capabilities to shape shift or change their eye color.
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