I don't get this at all.
The beginning of Rivera:
Demons invade the realm of the gods.
Gods can't win.
Gods create Grim Angels.
Gods win, but are sealed away themselves along with the demons.
1000 years later, the demons begin to awaken...
Maybe some of that is wrong, so please correct me.
But anyway, who from Yggdra Union are the gods, and who are the demons?
So you're saying the YU ending where
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Yggdra kills god
is the canon ending, and Yddgra and friends are the demons from the Riviera opening?