Choronzon is mentioned by occultists in the 16th century, but became famous due to Aleister Crowley and his belief system, Thelema, described further in his book "Liber AL vel Legis" or "the Book of Law". It is also called the Dweller in the Abyss or the Demon of Dispersion, and held vast amounts of knowledge. In a ritual with his follower Victor Benjamin Neuberg, Crowley claimed to have summoned Choronzon within a seal in the Sahara Desert, but could not control it.
"Origin: England. A demon summoned by Alistair Crowley. Crowley was supposedly attempting to gain wisdom from the 11th order of demons. However, the summoned Choronzon inflicted severe suffering on Crowley's disciples. Crowley was later said to either succeed in gaining wisdom, or was possessed instead."
"A demon that was summoned by a sorcerer named Crowley at the beginning of the 20th century. When he lost control of the demon it possessed him. It is the last hurdle in conquering the dark side of the ego."
—Shin Megami Tensei III: Nocturne Compendium
Choronzon's design is a recolor of Legion, although it is given a ghastly glow as well as being blue.
"A demon with a vast array of knowledge that serves sorcerers. It is known as the "Abyss," a formless aggregation of consciousness. It guards the door to all the knowledge of the cosmos. However, it is also the natural enemy of sorcerers, an evil being that sows chaos and contrives to send those who seek true wisdom to their downfalls. The great sorcerer Aleister Crowley is said to have actually summoned Choronzon. While Crowley succeeded in calling Choronzon down into his body, he could not control it, and though used many tricks to try to lure Crowley's follower Neuburg into breaking the seal and freeing it, Neuburg was able to resist. Sources vary on whether Crowley actually obtained knowledge or remained possessed by Choronzon."
—Shin Megami Tensei IMAGINE Compendium
Choronzon can be contracted in Ichigaya during the in-game night and most cars in the Old Tokyo Metro. It appears in the boss rooms of the bronze and silver level instances of Old Ichigaya Camp.