Fighting with the forbidden item, the Hazard Trigger, has transformed Sento into a dark version of Build, known as a “Hazard Form”. If he continues to fight using the Hazard Trigger, according to Katsuragi’s words, he will lose his sense of self. Sento is using the same technology the enemy has been using on himself! But Build loses consciousness during the fight. And then, Build stands before the Hokuto Trio as he decides to finish his fight against them… (Source: kamenrider.fandom.com)