Warning: This contains spoilers for the Jujutsu Kaisen manga not yet covered in the anime.
In the world of Jujutsu Kaisen, Satoru Gojo reigns supreme as one of the strongest, if not the strongest, Jujutsu sorcerer in the world. His Six Eyes and Limitless technique are some of the most deadly techniques known in the Jujutsu World. He can easily take over the world if he wants to.
That being said, what most people don't know is that there was a time when Satoru Gojo was almost killed in a duel. What makes it even more surprising is the fact that the man who almost killed the greatest Jujutsu sorcerer in the world is an assassin with zero cursed energy, and also no cursed technique whatsoever. The name of that assassin is Toji Zen'in. The following information is based on the manga version of Jujutsu Kaisen, so if you only watch the anime, spoiler alert!
Who is Toji Zen'in?
As the name suggests, Toji Zen'in was a member of the prestigious Zen'in clan. However, unlike the rest of the Zen'in who possess powerful cursed technique and an abundance of cursed energy, Toji had no cursed energy whatsoever, which means he also didn't have any kind of cursed technique.
In actuality, Toji was born through something called a Heavenly Restriction. This means in exchange for having no cursed energy at all, Toji was granted a near superhuman strength. His physical ability easily surpassed even those of first-grade or even special-grade sorcerers.
However, as a clan who pride themselves in their powerful and unique cursed technique, Toji was never really welcome in the family. As a matter of fact, he was often ostracized and treated like an outsider. This is one of the reasons why he decided to separate himself from the Zen'in clan. He even went as far as using his wife's surname and discarding his own family name. That’s why when Toji confronted Satoru Gojo, he used the name Toji Fushiguro.
His Life as an Assassin
His terrible upbringing in the Zen'in family and his own warped personality enabled Toji to be an extremely ruthless assassin. Not only that, the fact that he is a man with zero cursed energy and an absurd physical prowess due to the heavenly restriction makes him an extremely effective assassin, especially against Jujutsu sorcerers, which is why he earned the nickname "Sorcerer Killer".
Toji has been shown to use conventional weapons, such as guns and katana. However, when he is out on a mission to kill a Jujutsu sorcerer, he will use various cursed weapons instead. Those weapons are stored inside a cursed spirit that can decrease its own size to the size of a coin.
As a person with heavenly restriction, his five senses are tuned to the highest sensitivity to the point he can even feel any slight changes in temperature or air movement. This is also the reason why he can see and use his cursed spirit storage despite not having any cursed energy. His eyes are simply too sensitive that it enabled him to see cursed spirits.
That Time Toji Almost Killed Satoru Gojo
Toji has killed countless Jujutsu sorcerers throughout his life as an assassin, but the highlight of his career has to be the time he almost killed the strongest Jujutsu sorcerer in the world, Satoru Gojo. Not only that, he also defeated Suguru Geto, another special grade sorcerer, at the same time.
It happened during the time when the young Gojo and Geto are tasked by the Jujutsu High to be the bodyguard for a girl that is destined to be the Star Plasma Vessel, a being that supposed to merge with Master Tengen in order to strengthen and stabilize Tengen's powers.
Toji managed to sneak from Gojo's six eyes and stab him from the back. After that, no matter what kind of limitless techniques that Gojo throws at him, Toji has the perfect answer for them. He has prepared the necessary cursed weapons that can deal with Gojo's supposedly unbeatable techniques. The fight ended with Toji slashing Gojo's throat and stabbing his forehead. So yes, Toji has in a way killed Satoru Gojo.
After that, he proceeds to kill the Star Plasma Vessel and defeat Suguru Geto in mere minutes. Unfortunately for him, the moment Toji slashed Gojo's neck, Gojo immediately focused on doing a reverse cursed technique and started healing himself. Had Toji cut his head off, he would've won. But instead, that experience awakened the hidden powers in Gojo and allowed him to blast the secret Hollow Technique: Purple to the unprepared Toji and kill him.
Toji's Relationship With His Son
As you might've probably guessed by now, Toji Fushiguro is the father of Megumi Fushiguro, Yuji Itadori's friend and classmate at Tokyo Jujutsu High. It is unclear why, but not long after Megumi was born, Toji decided to leave his family, which is why he never sees his son's face, nor Megumi ever knows what his father looks like.
A battle-crazed man like Toji can never be a good father, so he had intended to give his son back to the Zen'in because he believes they can support and teach him better than he ever could. He knows the Zen'in are a wary bunch, so rather than simply giving Megumi to them, he asked for financial compensation to make them think he just wanted to sell his son. But just in case, in his dying breath, he also told the existence of Megumi to Satoru Gojo.
During the Shibuya Incident arc, a Jujutsu sorcerer manages to summon Toji's soul into the body of a living man. What they don't realize is the fact that Toji is so strong he ends up taking over the man and effectively revived and becomes a killing machine. While in this state, he accidentally meets and fights Megumi. Needless to say, Megumi doesn't recognize his own father.
Upon realizing that he is fighting his own son, the first thing he asks is Megumi's name. When Toji hears that Megumi still uses the name "Fushigoro", he smiles and says, "Good for you", as he stabs himself in the head. Thus ends the life of one of the strongest men on earth who knows nothing but hate and violence throughout his life. But in his final moment, he chooses to die rather than hurt his own son.