Thallo was a clone created by the Tleilaxu Masters in a laboratory in Thalidei on the planet Tleilax, during the time of the Ascension of House Atreides. His upbringing and training was led and guided by the Tleilaxu Researcher Dr Ereboam.
Kwisatz Haderach?
The Bene Tleilax had always believed in the advent of the super-being, the Kwisatz Haderach, who would ease humanity into the next stage of evolution. After hundreds of years of experimentation, the Tleilaxu created twelve clones of a near-perfect physical and super-intelligent male human beings, which they named Thallo.
They awakened one of the clones of Thallo in Thalidei, and told him of his destiny to lead mankind. Thallo, having the ability to see the future through prescience, knew that he would fail, and that his future was hopeless.
The Fenrings and Death
When Count Hasimir Fenring, his wife Margot Fenring, and adopted daughter Marie Fenring, visited the Tleilax to learn about the concept of twisting personalities; Thallo took Marie as a type of friend.
In 10,198 AG, he confided in Marie about his training and abilities. He also told Marie about how the Tleilaxu Masters has put memory cells of famous people of human-kind's past into his brain tissue, to help his cognitive reasoning. Such people included the founder of the Mentat School, Gilbertus Albans; Crown Prince Raphael Corrino I, and Jool Noret, of the Swordmasters of Ginaz.
Thallo also told Marie of his the concerns about his future, and after completely blockading himself and her in a sealed atmospheric control bunker under Thalidei; his misguided intention to commit mass genocide and suicide. As Thallo prepared to flood the city of Thalidei and the laboratory with deadly nerve gas, Marie was left with no choice but to show her skills as an assassin. She immediately killed Thallo, and the Tleilaxu Kwisatz experiment was discontinued.