This quest will trigger as a kingdom event near the end of the main quest Troll Trouble. Tristian will request an audience with you in the throne room of your capital. Once there he will inform you of a potentially dangerous Goddess cult which has apparently cursed the barony. Tristian wants to deal with the cultists directly, and asks that you meet him in the main square of your capital.
From the entrance to your throne room, head southeast beyond the tavern to find Tristian speaking with a woman named Amalia. Join the conversation by interacting with Amalia and learn more about her intentions. Regardless of your dialogue choices, the outcome will be the same: Amalia will invite you to meet with the Kingdom of the Cleansed cultists at their next meeting. Ending the conversation in a peaceful manner will result in 180 Experience being earned.
Note: You will have an opportunity to refine a Chaotic Evil alignment by killing Amalia after the conversation. However, to get the most out of the quest, this is not recommended.
Once Amalia leaves, you can speak with Tristian to confirm the next steps. After this the journal will update, and you will be prompted to attend the cultist meeting alongside Tristian. Be sure to include him in your party upon leaving the capital to ensure the best outcome is achieved. Once you exit Tuskdale, you will notice a new area on your map to the southeast, named Secret Sanctum. To reach it from your capital, take the following directions:
Tip: You will need to ensure that it is night time before entering the Secret Sanctum. If you enter during the day you will find nothing in the area. Any time after 23:00 will do the trick.
You will enter the Secret Sanctum from the southeast. From here, head northwest until you encounter a sacrament underway. You will need to wait while the cult leaders address the others, only to find out that the intended sacrifice for this sacrament is none other than yourself! There’s no two ways about it: you fight here or you die. Make your dialogue choices according to your alignment (earning you 225 Experience), and prepare for the fight ahead.
Having noted the departure of Amalia, who seems to have been unaware of the plans of her father, you will be forced to defend yourself. There are a lot of cultists (20) to contend with here, but most have poor defences and can be cut through easily. That being said, it’s worth buffing your party as soon as you enter combat, and charge immediately into the fray, focusing your melee on ranged opponents, to avoid any unnecessary initial damage.
As in most situations where ground needs to be won in a short period of time, Haste will be a useful buff here. Be sure to use it first, followed by anything available to your spellcasters which might further enhance the combat capabilities of your party, such as Inspire Competence and Inspire Courage for multiple party members, and Barkskin or Herosim for your tanks.
If you’re taking a lot of damage in a short amount of time, Tristian’s Channel Positive Energy - Heal Living will prove extremely helpful. For any targeted damage sustained by your tanks, keep them above water with a combination of Cure Light, Moderate, and Serious Wounds. In reality, you shouldn’t take too much damage so long as you focus on ranged units first, and once you’ve defeated enough cultists, others will begin to flee.
the last cultist you will have to contend with, and who most certainly will not flee under any circumstances, is Ruthgert, The First Faithful, father to Amalia. He’s much tougher than his followers and will take a more sustained attack to bring down. Don’t bother attempting to attack him first, as he buffs his followers and frequently heals them, using similar spells to Tristian. Aside from heals and buffs for his allies, Ruthgert doesn’t have much up his sleeve when it comes to inflicting damage. He will fall easily once his followers are dispersed.
When Ruthgert teleports away from the battle, a dialogue will open with Tristian. At the end you will have a choice between leaving the cultists who fled to their fate, or hunting them to extinction now. The former is a Chaotic Good action, while the latter is Lawful Evil. Whatever your decision, you won’t be seeing them for a while yet and you will earn 270 Experience after making the choice.
Loot the area and then return to your capital to speak with Tristian. You’ll find him in the northeasternmost point of the main square. Tristian will update you on the status of the remaining cultists and the quest will complete, earning you 900 Experience.