Queen Brahne was originally a just and fair ruler but, when her husband died, something inside of her changed and, due to manipulation by Kuja, she began to hatch a scheme to wage war against the neighboring countries of Lindblum and Burmecia using an army of mechanical Black Mages.
When Kuja told her that Princess Garnet possessed the power to summon powerful beings known as Eidolons, the queen's inner desire for power grew out of control and consumed her. Thus, Queen Brahne ordered her court jesters, Thorn and Zorn, to extract the Eidolons from Garnet for her own use.
Once Queen Brahne had gained the ability to summon Eidolons and had a sufficiently large army of black mages, she declared war on the rival kingdoms and ended up destroying both Burmercia and Cleyra. However, the power-mad queen made a mistake when she tried to usurp Kuja himself and he used the Bahamut she had summoned to destroy him on her fleet- destroying it almost instantly. Queen Brahne managed to escape, but was badly injured. As she began to die, she started to change- reverting back to her previous nature of a peaceful woman and begs her daughter's forgiveness for all the wrong she had commited.
Despite the horrible things she did, Queen Brahne was fondly remembered by her people, who built a tomb for her (as was customary for honored royalty in the past).