There's a half-eaten gnome in the area, one that the insane drow guy apparently was fed when he was nursed back to health by his 'friends'. That could be her. But it would have to be relatively new. On the other hand the stuff with the three drow seems like it's written as having happened quite some time in the past (the deal with the memory stone fading, for instance) The timeline of a lot of the stuff in the underdark seems a bit confusing to me, to be honest.
Building a time frame has been difficult, both on the surface and in the underdark.
We know that Lenore left her tower somewhere around 11 years ago.
We know that whatever happened at the Blighted Village happened about 120 years ago.
I think these are our two key time frames.
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So I would guess that the Sharrans were occupying the Grymforge area around that time (120 years ago), but something attacked them. Something large and specifically from the hells.
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But I don't think we have reason to believe the Sharrans built the Grymforge area. Sounds to me like they just took it over when they found it.
Meaning it predates the Sharrans, but I don't think we have any evidence as to who built it originally.
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Regarding the Shadow Cursed lands... I feel like that probably happened in the time of Ketheric, right? Which would have put it in that 120 or so years ago time frame.
There's a note on the Druid table in the grove that speculates about the Shadow Curse, suggesting that it was probably created by a deity, and most likely by Shar herself.
So. Maybe something from the hells attacked Shar's base of worship and in retaliation, Shar unleashed the Shadow Curse. Maybe to trap whatever demonic force attacked her followers?
--whatever event this was, it has to have something to do with the birth of the Absolute. Doesn't it?
--so my rough theory is that, for some reason, the Absolute attacked Shar about 120 years ago. In return, Shar unleashed the Shadow Curse. And that's been holding the Absolute back somewhat?
--now, with Lenore's groundbreaking research at hand (which started just over 10 years ago), something new is happening, and whatever that is, it has to do with the PCs and the tadpoles.
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Something I want to point out about the dream persona versus the tadpole.
In another thread, I suggested that I didn't think they were the same entity. I feel like a lot of people take it for granted that they're the same, the tadpole and the dream figure.
I'm not so sure.
For one, you still have the dreams even if you wear the Psionic Protection ring.
Besides that, the dialogue gives me the impression that the dream figure and the tadpole are actually at odds, as if the tadpole were trying to influence you to hurt the dream person.
And finally, when the dream persona shows you what is to come, what do we really see?
We see Tav fight an armored figure. Is this the same armored figure as one of the Abolute's Chosen? The armor looks admittedly less stylized, but very much the same otherwise, including the purplish color choice underneath. And as far as overlooking a burning Baldur's Gate, nothing says Tav created that. Shadowheart suggests that she was the conqueror in her dream, but all we know from Tav's dream is that he sees the city burning. For all we know, he's there to save it.