Fanfic Idea! (Could be lucemond, where Daemon long prepared for the betrayal of the greens and urged his children to make alliances of their own, and Lucerys' anger has reached his endpoint)
Lucerys is pissed. He is angered, he is tired, his sorrow for the loss of his sister and the rage he felt for her undue death was what made him storm through the Baratheon home. He already knew his uncle was there. He saw the decaying dragon before he even got to land.
He knew him being there wouldn't do anything. His grandmother already told him how different Borros was to his father, but he had to at least pretend to try, for his mother. So there he was, dripping wet from the storm, hate and anger in his eyes, but a smooth, calm face, as to not give the people more reasons to look down on him. He deigned to smile at the girls, they were his kin, after all, and someone who he had been sending letters to ever since they all met, a secret from Borros, lest he tries to force one of his girls to marry him. They, in turn, smiled back.
He didn't bother looking at his uncle Aemond, barely acknowledged his jibe. He knew being ignored would piss him off more than any answer he would ever give, and that was more than enough of an incentive to pretend he was nothing more than passing wind from the storm, loud enough to howl, but something easily ignorable. He simply came to deliver the letter. He already knows that man wouldn't budge, had already planned to marry one of his girls to Aemond, or whoever the greens were willing to sacrifice. To him, Borros was already a traitor and an oathbreaker, and him being there was just a farce.
When the obvious refusal from Borros came, coupled with a few insults that made Lucerys momentarily enjoy a vision of just ordering Arrax to enter the hall and fry him before his courtiers, he nodded, and told him he would take his message to the queen, before turning around, his head held high. He nodded once more to the girls, and they knew. It was time to accelerate the plan. Borros is to die as soon as Aemond leaves. He will drink his ale, swallow his meat. The story would be that he saw the ghost of his father, angered by Borros for making him, making his house, bear the name oathbreaker, before forcefully taking his son's soul, leaving behind his frightened body as a warning. That way, it would be seen as a horrible omen, Aemond would only find out once he reached King's Landing, and would be too late to return.
In reality, Floris is to drop a special little medicine in her father's drink, one made by Alys, that causes people to have frightful illusions based on things they have been thinking about. Eventually, they becomes so stressed from fright, they started trying to get away from whatever apparitions their mind created, and finally, die from their heart giving out. He has already made sure the main thing on that man's mind would be his father's vow to his mother, from the letter. If things go as planned, (and he ensured it will, seeing as he gave a stronger mix to Floris) the eldest, Cassandra, is to be the new lady Baratheon in his place, and he knows she would support them. The greens would call them traitors, oathbreakers, but then again, Cassandra can claim she was undoing her father's oathbreaking by following her grandfather's vows, so his soul can rest at last. They can call for foul play, but could they when the man dies right in front of all his men, loyal or otherwise, watching him run from someone who isn't there, watching him scream for his father to not hurt him, before succumbing to his death, as if Lord Boremund snatched his soul from his body to ensure he never breaks another oath?
He wonders if any powerful house would try to side with the greens with the threat of their previous heads coming to haunt them to their death for oathbreaking. He thinks not, as the people of Westeros seems to hold great importance to the words of those long dead. If they do, well.... he has already made quite a lot of connections. It wouldn't be hard to ensure someone's predecessor would visit them to give a "stern warning".
Before he could leave the place and lead poor, wet Arrax out of the Storms, Aemond decided to order him to wait, to claim an apparent "debt". Lucerys was just going to ignore him, but his anger filled him. They were already stealing his mother's throne, and their own inheritances, simply because their mother wasn't a man. They have already taken their mother's chance to say goodbye to their grandfather, not even allowing her to burn his pyre. They have already killed his sister, who they didn't even allow to breathe. If there was anyone here that had the right to demand for someone to pay a debt, it was him, his family. Not Aemond.
And he will get revenge. Just not now. But he will not allow Aemond the chance to call him a coward. Instead of facing Aemond, he looked back to the soon-to-be-deceased Lord Borros.
"My Lord, I know it isn't my place to tell you how to run your house, but I do strongly suggest you close your windows and doors."
There was confusion in the crowd, even Aemond looked momentarily confused at his statement.
"The howling of this storm is quite strange. The wind just now sounded like it was trying to say something."
With the people realizing what he was implying, and his uncle's face going red, (with embarrasement or anger, who knows, who even cares?) he walked away calmly as the guards prevented Aemond from chasing him, his screams for his eye ignored once more. Lucerys, after all, has more pressing matters to attend. He is to fly to a cave one of the girls secretly found. There he would wait out the storm, wait out the shocking death of Borros, before "flying back" to Storm's End due to a "premonition" of sorts, and return home with the the Baratheon alliance in his pocket.
Plotholes? Yes. Why? Because tired, can't think much, school sucking the life out of me.