When Aleph encounters Crowley, he is angry that Lucifer will not allow him to have Sabbaths (orgies involving demons) and takes this frustration out on Aleph in the form of Master Therion. He is eventually defeated.
"The name of the beast that the mystic Aleister Crowley called himself. It refers to the beast that appears in the Book of Revelations. According to Revelations, Therion is the enemy of humanity, a beast with the power and authority of Satan. The beast is said to come from the sea, having seven heads and ten horns, a body like a leopard and legs like a bear. Another beast with lamb's horns is said to come from the earth, forcing man to worship the beast from the sea. Everyone must also bear the mark of the beast, the number 666, on their hand or forehead, or they cannot buy or sell goods."
—Shin Megami Tensei IV compendium
Master Therion can be found in the Monochrome Forest within the Expanse. He can teach Flynn the Agidyne and Concentrate skills through his Demon Whisper.
He is also a boss in the New Game Plus Challenge Quest, Stop a Summoning. Therion is in charge of the ritual and is planning on holding a Sabbath in Shibuya 108 in the world Infernal Tokyo's Shibuya. Therion seeks to summon the scarlet woman by having the Demonoids and humans have pleasure. He seeks to become one with her and reveals his physical form, speaking of how he is intertwined with her. Knowing that Flynn was not part of the ritual he informs him that Master Therion has the right to do whatever he wishes, and that he will slay those who get in his way.
Master Therion commands Flynn to die as the battle starts and states he will show him why others fear him and tremble at mentioning "the Beast." Surprised by his own defeat and impressed at the same time by the skill Flynn displays. After the battle he laments how he was so close to reaching the great mother and that she is lost in another dimension, unable to return to the Expanse. Master Therion decides to leave since he no longer has a purpose here, disappearing while those who attended the Sabbath leave. Master Therion appears as a regular enemy in the Monochrome Forest.
Master Therion can be found in Camp Ichigaya. He can teach Nanashi the Makajamaon, Agidyne, Concentrate and Mabufudyne skills through his Demon Whisper. He benefits from learning Fire, Ice, Darkness and ailment skills.
Master Therion is a boss in Ice Book. An angel of Muspellheim, he is the demon who disguises himself as Zebul after Narasimha and Ictinike imprison the real one. He will later guard one of the treasure pieces needed to enter Shugoshin's realm. Upon completing the story the revived Master Therion can be found in the Black Forest.