I'm trying to think of a reasonable speech to get her to come with us, but I keep running into walls...
As far as I can recall there's not really much we can say on that score...
I mean, her main motivation is the punishment of those who committed the atrocity that was the murder of Kos, the village that worshiped her, and the orphaning of her offspring.
If what I can recall is correct, Kos had an Ebretas like relationship with the village, they were on cordial terms, at least cordial for an old one and a bunch of humans.
The church, being bastards as we all well know and I have pointed out numerous times before, decided that they wanted the knowledge, power, and other assorted good and plenty for themselves.
So instead of asking for it, trading for it, or simply going there and talking to folks, they killed everyone, killed Kos, and took what they wanted from her corpse and the village for study.
That's what the previous area was for by the way, the area where they conducted their Tuckeresque style experiments.*
The beings there are either researchers who delved way to fucking deep ala Bergenwerth, experimental subjects ala the healing churches usual Umbrella style guide to scientific research, or blood drunk hunters looking for something else to kill.
She stays where she is so they can't progress further into the nightmare and return to the village.
If it's to guard the tomb and resting place for those damned souls, to keep them from Kos remains that reside in the nightmare, or just to keep them away from the Orphan I can't really say.**
All I know is they aren't getting there, and by extension we aren't getting there, unless she lets us, or we kill her.
The best outcome we can probably hope for here is just for her to let us pass.
*Honestly the more I think about it, that muderous fuckwit would blend right in with the church. I mean what doesn't say progress like turning your wife and child into abominations against god and man that live in perpetual suffering and anguish?
**It should also be noted that it appears she died after the incident with the village, and it appears it might have been suicide, considering her throat was cut. Of further support of this theory was the fact that she was so disgusted she threw away her prized swords into the well of the village, unable to stomach the sight of them. Take from that what you will...