Read the Famitsu article. Apparently they are basing it on the Book of Enoch, and are following it closely.
Origin of Book of Enoch:
The Book of Enoch is considered non-canonical by both the Jews (Judaism) and Christians. It is proposed that Judaism rejects it because it prophecies about Christ. Christians consider it non-canonical because it a nutshell, there isn't sufficient evidence for it to be considered canonical. (All writings that go into the Bible is subjected to a surprising amount of scrutinity.) Popular opinion is that it is a fictional work in the late BC period based off bits of other writings.
(spoilers aplenty below - but the knowledge is available in the public domain anyway)
The Book's story:
In a nutshell, a group of angels named the Watchers were supposed to watch over men. But they lusted after the daughters of man, and 200 of them descended to take wives for themselves and have sexual intercourse. They led mankind into corruption, fornification and all manners of evil. By God's judgement they were cast into the darkness, and the Great Flood was sent to wipe out all their evil offsprings (monstrosities) and all men that chose their ways. 10% of them were allowed to remain in the world as disembodied spirits to test the sons of man - and are then named "demons". But they shall be cast into the fire in the Final Day. The Book ends with prophecies of the coming of Christ and the Final Days.
In the book, the man Enoch plays the role of a witness of the events, prophecises of the Final Days, and warns Noah of the coming Flood.
The game's story (according to Fami):
Seems to follow the Book of Enoch closely, but omits the sexual intercourse part. In this interpretation Enoch is presented as a somewhat angelic entity (info unclear), and is the one tasked with the mission of casting the fallen angels into the darkness.
For more info do a search on Wikipedia for Book of Enoch.
