Betta, Liesel and Yves are fleeing Orlais because of the Civil War. I think Liesel is Bettas lady in waiting, but she could be her sister, servant, aunt or friend. Yves is Liesel's baby boy. The trio flee Orlais and while running they find an abandoned house in the forest with a demon inside it. (I think the house Chateau d'Onterre and the demon is the friend from that quest.).
The demon makes contact with Betta, pretending to be her mother.
I'm guessing that the demon messes with Betta's mind like the Desire Demon did with the Templar in the Circle of Ferelden. Making Betta see what the demon wants her to see and do things the demon wanted her to do. So Betta might think she's helping someone bathe while she is in fact drowning them. I think the demon might be a desire demon, fueling on Betta's desire for the simpler times when she was a kid, while mother was alive and there wasn't a war going on. Which is why the journal entries become more childlike after they find the house.
Anyways.The demon convinces Betta to trap Liesel and Yves on the second floor. [Liesel and Yves have been gone a long time.-snip- Mother made a bed for them upstairs].
Then the demon makes Betta kill Yves, fooling Betta that she's helping Yves sleep. [Young ones need their sleep.]. Liesel either witnesses Betta kill him or finds Yves body, freaks out and tries to escape the house or hide herself. So the demon starts a demented game of hide and seek [Mother and I played a hiding game in the house. I found Liesel! She didn't like it when I found her. She was screaming about "Yves"].
The hide and seek ends with either Betta or the demon killing Liesel. [Mother told Liesel to be quiet; she was ruining the game].
Now Liesel and Yves are dead, leaving the demon and Betta alone in the house. Since the demon has no one else to torment, it turns it's eyes to Betta, playing on her guilty feelings. [Mother says I bore her. -snip- She got sicker because of me.] Eventually the demon tells Betta to go and gather Crystal Grace by the waterfall, either knowing that she'll fall or it convinces her to jump. Or just flat out pushes her to her death.