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When you visit the Cursed Abbey, you might find a book there detailing the three Holy Weapons, and how you'll have to sacrifice two in order to use one. Here's where you can find the three weapons.

Dagger: In Rob's room in the Thieves' Guild in Verdistis.
Sword: In the Cave of Illusion in Iona's Dungeon (Level 1).
Amulet: In the Burial Chamber in the Dark Forest.

Once you have all three, travel to the Holy Weapons Chamber located northeast of the church in the northern part of the dukedom. There stairs to the chamber are hidden among the bushes.

Inside you'll find a statue, two circles and four tombs. Don't touch the tombs or else you'll break the quest. To start the ceremony, stand on the eastern circle. When you pick the holy weapon you'd like to keep, it will appear in the western circle. Whichever object you decide to pick, chances are it will be very powerful.