NaNo Prep 2023 Day Two - Plot premise

The story takes place in an alternate world, in my mind. I had not thought of a time range. It might be the 1700s? I am not 100% sure of this yet, but that seems about right.
The protagonist is Icerna Frostbane. She and, eventually, her niece Lysandra Frostshade will work with Ignatius Flamethrower to overthrow the oppressive regime of Lord Malachar Shadowbane.
Icerna’s major flaw is that she is too trusting. She trusts that Lysandra is working with her when, in fact, Lysandra is working against her. A typical teen, Lysandra simply wants to be in with the “in” crowd, which is definitely not a bunch of old mages. The need to fit into a world where she does not truly fit is Lysandra’s biggest flaw.
Of course, Icerna wants to overthrow the oppressive and evil regime. But what is her motivation at this particular time? There must have been some impetus. An impetus would be someone being killed by soldiers of the regime. An impetus might be someone being imprisoned unjustly. Does she perhaps find out that Aelar was imprisoned and did not actually leave her as she thought? But how would she find that out? And I want her to only know about that after she is imprisoned herself. So, the impetus must be something else. It will likely take a new character I have not invented yet.
Icerna wants to live in a world where magic is accepted, as it was in the ancient years. But the new regime does not tolerate magic.
Maybe it was something that happened to Ignatius’ family that brings about the need to move quickly. His family was taken away, perhaps, while he was gone on a quest to somewhere. He came home to an empty house (I know I have used this trope before, so it feels a bit trite). He is an older fire mage, so I think it will have something to do with his son. His son is taken away and, he fears is dead. Maybe he was one of the youth in the group Lysandra is now running with. This would be an impetus for both Icerna and Ignatius to get involved. But that won’t work, either, because Icerna cannot fid out that Lysandra is not on her side until later in the novel.
So what is the impetus for Icerna and Ignatius to get involved? Rumors flood through the mage community that Lord Shadowbane is now having all mages rounded up. They need to revolt against the regime before they are sent away themselves. As the oldest fire mage and ice mage in the kingdom, they are charged with mounting the rebellion. I think this is the impetus.
The antagonist if Lord Malachar Shadowbane. He is the current ruler oppressing the people of (whatever this kingdom/land is called). He has called an end to all magic and has prohibited any and all mages from entering the kingdom. Those already there have hidden their magic in order to stay free people.