Talk to Ser Maximilian Eizenstern, who has been appointed Captain of the Hunt by Duke Edmun Dragonsbane. Make sure you carry the Wyrm Hunt License while talking to him. The Wyrm Hunting License you carry permits the bearer to undertake the most vital of tasks: Those missions given by Duke Edmun himself.

Ser Maximilian presents you some tasks that need attention, such as:
Decipher a text
Rout a monster infestation
Investigate a cult
Aid a research team

Choose to investigate the cult.

Maximilian tells you about a society called "Salvation". They are a band of heretics who worship the dragon, and welcome the perdition it brings. In the chaos that came with the Wyrm's arrival, their influence has grown. Maximilian asks you to keep watch for signs of Salvation and report to him if you find out anything.

This starts the quest:

Seeking Salvation
Learn more of Salvation

Objective
Wyrm Hunt Quest
Learn what you can of Salvation, a secret society that worships the dragon and sows chaos.

First, go to the Inn, just east of the Fountain Square. Talk to Meridith inside. He refers you to Mason (the guy who greeted you when you first came in to Gran Soren). Your goal now is to track down Mason, who seeks his own answers about Salvation.

Go to the aqueducts to the south and climb down a ladder, enter the slums and search for Mason there. He gives you a Salvation Badge which will identify you as a Salvation Member. You now need to renew your inquiry into Salvation. You will make use of the Salvation badge Mason entrusted you to uncover new leads into the cult's motives.

The map will now have new red markers. Patrol the streets of Gran Soren, speaking to all of them. Baden, near the Arsmith's Alehouse tells you that he did not expect the Arisen to share the Salvation's ideals.  Amity, nearby, has more information, about a meeting that will take place. A guard named Ser Sairus, on one of the streets behind the inn, has more details about the gathering. He tells you it will take place down in the catacombs.

You now need to search the Catacombs. The entrance to the Catacombs is actually outside Gran Soren, a bit to the northwest. Travel there. Enter the Catacombs. Light your lantern. Explore these tunnels. There is a locked gate, you have to go around. There are rats in this dungeons, as well as lots of undead. Holy damage is good against these. Some coffins/sarcophagus here are booby trapped. If you notice blue smoke coming out of a coffin when you open it, move back immediately, or you will become cursed. Or better yet, do not open cursed coffins, especially if you do not have the curatives or other means to cure the curse status.

Soon you reach a lift with a lever next to it. The gate you saw when you entered the dungeon, is nearby so open it from the inside before activating the lever, so that you have a shortcut for next time you come. Pull the lever, and your party goes down deeper under the ground. Work your way to the meeting room, fighting more undead, and even a Cyclops, along the way. Follow the voice of Elyision, the leader of Salvation. The voice will get stronger as you approach your destination.

Enter the gathering hall. There will be a brief cutscene. Observe the Gathering. Soon you will be spotted. Watch some more cinematics, as undead kill all the cultists and zealots. Once the cutscenes end, defeat the undead. Then, pursue the Elysion through the western door. Climb up some spiral stairs.  You run into Mason who stopped the Salvation leader.

You now get a new quest:

Justice Done
Decide the Leader's Fate

It sounds like Mason wants you to assassinate the leader. But the leader, Marcelo, asks you to spare him and promises a reward of gold for sparing his life.

Kill the leader or not?
It is better to KILL the leader. If you don't kill the leader, Mason will kill him anyway. Mason will also be very upset at you for not killing the leader yourself, and he will not have any more quests for you. So storywise and questwise, it is better to kill the leader. Though you can choose to let him live, and you will still be able to beat the game, but won't have access to any of Mason's quests.

Killing the leader causes Mason to trust you, and he will give you additional quests later in the game.

Once you have killed (or spared) the leader, it completes the quest, Justice Done. Return to Gran Soren and speak with Maximilian to report your findings, thus completing the other quest as well, Seeking Salvation.