"I have been brought back into this world to lead it into the fiery gates of hell!"
After Nicolai made his soul pact with Astaroth, he was able to successfully contain it with his ego. After he and Yuri faced off in Apoina Tower, Special Agent Kato saves him and takes him to Japan. There he was put under the care of Dr. Hojo. Nicolai would be used for the Russian negotiations, but first Astaroth must be removed from his body. Hojo tried to remove it the first time, using a dog as a new vessel for Astaroth. The experiment proved too dangerous, and the dog was warped by Astaroth's power to become Shoki, which terrorized Nihonbashi before Yuri and his friends defeated it.
Afterwards, Hojo tried again to extract Astaroth. He tried a more drastic approach by digging into Nicolai's soul force. But Astaroth eluded him. Fearing failure, he proceeded until Nicolai's ego failed. Now Astaroth takes full control of his body. He escapes and heads for the Immortal Mountain, where he seeks to destroy the world.
After being moved to Mt Fuji by Kato and his Mutant Apes, he manages to briefly possess Saki Inugami and attempts to use her to kill Yuri and company. Now knowing where he is, the party heads off to fight him in the fiery bowels of the Immortal Mountain.
When they find him, he's encased in crystal and speaks telepathically, telling of how the age of mankind will end. After insulting him, Yuri and his gang prepare for the final fight with the true form of Astaroth, Neo-Astaroth.
"My soul has been revived! The age of mankind is coming to an end. Joy, sadness, desire. All of it is finished now! I will usher in a glorious new future."
Yuri: Before you do, can I ask a favour?
"What do you wish, human."
Yuri: I'd wish you'd shove it. You know where.
Astaroth ~ Battle with The Fallen Angel is the battle theme that is played when fighting Astaroth and Neo-Astaroth. Select sections of the music are also played in cutscenes that precede both the fights.
It's corresponding insanity track is "Crack Your Mind ~ The Fallen Angel Runs Wild".