Astorath was a demon from Hell who was the father of a human boy named Shannon Phillips. He also had a man named Nathanial Blaque assist him in his plans. First, Astorath caused everyone around the world(except Shannon, the Ghostbusters, Slimer, and Janine)to transform into demonic monstrosities that worshipped him like a god. After he made Blaque suffer in a cold area in Hell, the man detonated a hydrogen bomb, seemingly destroying both Astorath and Nathanial Blaque.
Astorath is a very powerful demon. He can vaporize humans or turn them into his own demonic goons. He also implies that he is more powerful than Gozer when he states that the god is a weakling compared to him.
Astorath is a misspelling of Asteroth, who is also a demon.