The Toll House/Karlach encounter feels incomplete. Was she supposed to have 1 horn/a more unique character? That seems possible, as game files indicate that she might be a companion. Anders and crew serve Zariel, which was a surprising twist, and ultimately I ended up fighting them, but the quest doesn't really end (I think she's supposed to join the party at that point).
5e does have a couple of "detect lie" spells - Zone of Truth, which forces people to speak the truth on a failed save, and detect thoughts (as it is difficult to lie without thinking about the truth you are concealing). Spells like Charm Person, Dominate Person, and Suggestion could also compel the truth, though often with negative consequences (when the spell wears off they realize that they've been manipulated).
I don't see a reference to all the possible ways you could investigate this conundrum listed here.
Did you, for example, try speaking to the dead people in the house, to see what happened there?
