The design for the character seems to be based on the Buddhist mummies, Sokushinbutsu. These were monks who performed self-mummification by starving themselves for about three years. At the end of these three years, they were entombed alive with just enough room to sit cross-legged. A tube was placed in the ground, leading to the tomb for air. The monk had a bell with him and every day, someone would listen for the bell to make sure the monk was still alive. When the bell no longer rang, the air tube was removed and the tomb was completely sealed. A thousand days later, the body would be exhumed. If it was preserved, it was revered as a god.
"Origin: Japan. A title given to a highly virtuous monk who has attained Buddhahood during his lifetime. Among those named as Daisoujou is the founder of Shingon Buddhism, Kuukai."
—Shin Megami Tensei GBA/iOS Dictionary
Daisoujou is one of three rare and difficult fiend bosses that can be found hidden throughout the game. If you beat him, he has a slim chance of dropping the Reaper's Bell, the most powerful Chaos-aligned weapon in the game.
"A monk who died while fasting for the sake of people. Because of his spiritual power, his body continues to exist without rotting. It is said that he will appear in front of people on the day of salvation."
—Shin Megami Tensei III: Nocturne Compendium
Daisoujou appears as a boss in the updated release of Shin Megami Tensei III: Nocturne, titled Shin Megami Tensei III: Nocturne Maniax. After the Demi-Fiend's meeting with Gozu-Tennoh, a soul in front of the Mantra Headquarters in Ikebukuro will mention a "strange monk" in the eastern part of the city. Should the Demi-Fiend then travel to that area, he will be summoned to another realm by Daisoujou, where the fight will take place. He will insist upon the futility of the Demi-Fiend's efforts, believing in the absolute power of the rules laid for humanity. Upon defeat, he will drop the Candelabrum of Eternity.
Afterwards, the Demi-Fiend will be able to fuse Daisoujou by using a Deathstone, an item found in the Labyrinth of Amala.
Daisoujou is a reoccurring boss within IMAGINE. He appears as the summon orb boss of Ichigaya Camp's bronze-level instance. During new moon, he can be summoned by players of the law alignment by entering certain regions of Ueno's Kuchinawa Cave. He can also be encountered through a D-Egg summon or in Nakano Stone Site's gold-level instance, where players can obtain the ability to fuse him. He is a triple fusion of a Mou-Ryo variant, Matador, and Yatagarasu.
"A monk who died while fasting. His spiritual power allows his body to continue to exist without rotting. It is said that he will appear before people on the day of salvation."
—Persona 3 Compendium
To acquire the Daisoujou persona, the player must perform a pentagonal fusion with Mithra, and the four Mitama Personas, Saki Mitama, Nigi Mitama, Ara Mitama, and Kusi Mitama to create Daisoujou. Fusing Daisoujou is also necessary to complete one of Elizabeth's requests.