Training, and Time on Tleilax
In Paul of Dune, by Brian Herbert and Kevin J Anderson, during the Ascension of House Atreides, as Margot was married to the sterile Count Hasimir Fenring, Marie was raised by him and her mother. The Fenrings trained their young child as both an assassin and a Bene Gesserit.
The Sisterhood was aware of Marie from her birth, but the Count and the Lady chose to raise her free from their tutelage. As her parents had their own plans for Marie to eventually seize the Imperial throne from Paul Atreides, the Fenring’s decided that Marie should be exposed to a variety of cultures. One of the first groups Marie was taken to live among were the Bene Tleilax, in one of their main cities of Thalidei. As the crisis of Mu’ad Dib’s Jihad reached fever pitch throughout the Old Empire, even the Tleilaxu were interested in Marie’s abilities, as a type of fulcrum against Paul's growing authority.
During the year that the Fenrings lived in Thalidei, Marie was introduced to the unstable Kwisatz Haderach experiment that the Tleilaxu were trying to create, the gholas named Thallo. The prime ghola candidate Thallo, became Marie’s friend and confident, and showed Marie his superior intellect in helping to manage the affairs of the Tleilaxu Masters of the holy city. Thallo eventually went “unstable”, as the Tleilexu Kwisatz creations were prone to do, and tried to poison all of the inhabitants of Thalidei. Marie was easily able to dispatch and kill Thallo, earning the respect of the Tleilaxu leaders.
Marie on Arrakis
Soon afterwards, at age six, the Fenrings declared her ready for her mission on Arrakis, and had Marie accepted into Paul's court as a playmate for his young sister Alia. It was discovered that Marie had an enhanced resistance to poison, as she and Alia played with scorpions on Dune. Generally, however, the time for Marie and Alia was typical for that of young school-aged girls, as they spent their days in games and learning.
Six months later, at a banquet with her visiting parents, the Fenrings executed their well-planned assassination attempt on Paul. Though security preparations were high at the banquet, Marie came within feet of murdering the Emperor. Alia managed to kill Marie first, and stabbed her deep with a blade, earning her the nick-name “Alia of the Knife”. Minutes later, Margot's revelation of her daughter's paternity surprised Paul enough to allow Hasimir to stab him mortally.
Immediately, Atreides’ concubine Chani gave Paul the poisonous Water of Life, which he used to slow his metabolism, and repair his body internally, causing him to survive. Instead of executing the Fenrings, he banished them to permanent exile on Salusa Secundus, to contemplate raising their only daughter as an instrument of death. They also had to live with the deposed emperor Shaddam Corrino, whom they hated and distrusted.