Diviner's Portent

An ability that tells you to roll a die will also specify what to do with the result of that roll. Most often, this is in the form of a “results table” in the card text.

Some abilities, like that of Pixie Guide and Barbarian Class, replace rolling a die with rolling extra dice and ignoring the lowest roll. The ignored rolls are not considered for the effect that instructed you to roll a die, and do not cause abilities to trigger. For all intents and purposes, once you determine which dice count, any extra dice were never rolled.

The instruction to roll a die and the effect that occurs because of the result are all part of the same ability. Players do not get the chance to respond to the ability after knowing the result of the roll.