Quest giver: Ser Maximilian
Experience: 15000
Gold: 20000
RC: 25
Investigate the Secret Meeting
Infiltrate the Catacombs
Vanquish the Skeleton's Master
Report back to Ser Maximilian
Talk to Ser Maximillian outside the duke's castle gates to start this quest. During evening, head to the first marked location and you'll find two men under the bridge. After the scene, approach them and one of them will bribe you. Resort to taking the key by force or giving the man 10,000G. Another alternate way of doing this is by wearing a set of Salvation Clothes and following one of the members as he hides the key in a barrel in the Alehouse. Regardless of how you obtained the key, head to the Catacombs.
I suggest entering through the main entrance in Deos Hills. Make your way to the previously locked gate where you'll see Tagert talking. Open the gate using the key you obtained and go upstairs. You'll find a large door relief that has five circular slots. To open it, you have to find five Gems of Salvation by opening all sarcophagi in the area.
Follow the link for the map where you can find the gems.
(Image added by AeonsLegend)
After finding all the gems and placing them on the door, it will open and you'll have to face the cult member / skeleton master named Balsac. He will be able to summon skeleton warriors and mages for him but he shouldn't be much of a trouble. You can concentrate your damage on him since the summoned skeletons will just crumble to dust upon his death. Report to Ser Maximilian to complete the quest.