This is the first ending I came across. If the player continues to the Otherworld (a point of no return) without acquiring a quest reward, the save file is doomed to Ending B. The final boss possesses the main character and condemns the rest of the party.

The fight against her is winnable. I suppressed her attack rating to stay alive and chipped away at her health. The outcome, however, is unchanged.

1:21 - Pre-boss fight sequence (Yorshik)

Custom pillarboxes made from https://www.davidrevoy.com/article753/hibiki