At the end of the second season of the anime, Seido Takizawa was almost killed. And in the begining of the third season, he appears as a powerful half Ghoul. What happened to him? How did Seido Takizawa become a Ghoul?
How did Seido Takizawa become a Ghoul?
Captured by Aogiri, Takizawa was given to Dr. Kanou and given the designation "OWL 15." Of the sixty-three investigators subjected to the ghoulification process, he was the only one considered a successful product. Some time after his surgery, he was imprisoned in a cell and visited by the doctor. Rather than answer his questions, Kanou removed his clothing and showed Takizawa his regenerating left arm. Horrified by the deformed limb, Takizawa began to panic while Kanou explained the stages of the transformation to him. As he began to vomit, he was informed that further tests would be conducted later and left alone in his cell.
In the time following his initial transformation, Takizawa was repeatedly subjected to extensive torture at the hands of Dr. Kanou. This process was intended to force the development of new Rc pathways, strengthened his body and removing any lingering human weaknesses. His torture would eventually shatter his mind and diminish his life span.
I'd also like to add something else, Houji made Tataras subordinates Fei and Yan his weapon aka a Qunique. Tatara took Houji's subordinate Takizawa and made him his weapon. An interesting little detail.
Source Takiawa became a ghoul