There should be a dialog when Leandra gets to 0 HP, where she says 'you can't stop me. it's too late, the void is here', then Icara breaks in and says 'this is the moment, use the spell'; Zandalor objects, Icara replies, then the main characters have the option to comply with Icara's wishes or not. There is a then a cutscene, a dialog with the sisters, and a choice about whether to bring them with you.
There aren't any choices in the initial dialog with Zandalor when you first wake up from the horror sleep.
