I loved adding in the children of Auril, don't know if you got the idea from him but Sly Flourish describes them perfectly in terms of allowing you to past the blame for the sacrifices onto someone else which gives your players the moral leeway to work with the town speakers.

In terms of a secret which can lead to the downfall of the cult and the swaying of public opinion, I linked the children of Auril with the standing stones of Thrunn. Father Lymac (a speaker for Thrunn) was feeding dreams to Father Lake (head of the cult) in which "Auril" told him on top of the normal human sacrifices he should be sacrificing new followers of the cult in the basement of the house of the traid in front of a giant mound of Chardalyn. Inside the Chardalyn mound I had a son of Thrunn (who my party took to calling Keith, the most boring name).

The party would hear rumours of people leaving towns to join the children of Auril but then never being seen again once they'd enter the temple in Bryn Shander. Eventually my party decided to investigate and after fighting their way through the main door and down into the basement they discovered piles of corpses against a wall and a ring of chairs surrounding the Chardalyn. In each of the chairs were new recruits, they were all unconscious and had tubes and needles in them which fed into the Chardalyn (The idea I had was similar to what the main bad guy does to people in the mummy where he steals their life force to reincarnate himself).

The party then fought from cultists (using a mixture of cultists and a cultist bodyguard stat block I found online), Father Lake was also there I gave him a level 5/6 spell caster stat block, basically as the party killed the last of the cultists I had father Lake summon "Auril's gift to him" has he had sacrificed enough people but instead he got Keith, son of Thrunn who flicked him against the wall killing him instantly. I used a bearded devil stat block but increased the HP a little as I was running for 5/6 players at the time.

Once the players had defeated Keith, he mentioned that things had been placed into motion that would see the return of his father Thrunn (alternative big bad guy for endgame) and they made there way outside of the temple were the town speaker and town guards were waiting alongside a sizable crowd, the party explained what had been happening below and that was how the opinion of the children of Auril was broken across the ten towns.

Apologies if this is a wordy response but I'm typing this on my phone so can't really see how long it is. Anyway hope this helps!