Jujutsu Kaisen has so far followed many dark and twisted paths along the way, creating both terribly heart-wrenching and horrifying gruesome scenes throughout. Many of these especially horrifying scenes even came as a surprise for fans, only increasing their shock.
In episode 23, fans learned the horrifying truth behind Fushiguro's backstory, specifically that he had been sold off by his own father. He met Gojo when he was young, who was to mentor Fushiguro at the academy in the ways of jujutsu sorcery.
Fushiguro acknowledges that in the first grade his father simply vanished without a trace, and he soon met Gojo, who was there to collect him. While Gojo doesn't say that his father sold him directly, Fushiguro is easily able to realize what happened nonetheless.