Since King Neville was the Head of the Council, he and his family met with delegations from other continents.
Lucille could have gotten acquainted with someone from Fynnia or Griffith through such meetings.
Perhaps, someone from them egged Lucille on and made her envy and jealousy over Arabella get worse. Until Lucille reached a boiling point and ended up trying to assassinate her.
Arabella had done this to a pest to her plans in her previous life. What if there was someone who had the same kind of mindset and did this method too?
And the ultimate goal was not only to kill Arabella.
Her death was simply a means to an end.
The real goal was to make Ferdinand disagree with joining the Council and cause chaos and distrust among Council members.
Moreover, Valeria would not take this sitting down.
They were invited to the Assembly by none other than King Neville himself. And yet his daughter attempted to kill Arabella.
It was a great insult to Ferdinand and Valeria as a whole.