D&D General Would You Play This Theme?

[Eltab]

And there's a prophecy that we'll be involved in stopping a mad Elder God from destroying the world. (This might be hampered a bit by the fact that the sorceress is secretly worshiping the Elder God.)

Is your DM around?
The sorceress is the key to the Elder Evil's plan to manifest in this reality. Doing this will destroy her utterly (even if she is a willing vessel).
The catch is that if the PCs can provide some other, more powerful more suitable vessel - like the Matron Mother - who will fight for her own survival and unleash the whole force of the House against the Elder Evil. We hope. At least, that's how I read the caveats and vague language in the prophecy.

And, it's a very Drow-like thing to do.