The Tainted Souls Quest/Spoiler

Requirements

 * Antlers
 * Holy Water
 * Star Herb

Obtaining the Items
First, you will need to obtain a Sacred Antler Talisman. To make one, you will need Antlers, looted from Deers; a Star Herb, looted from Witches or bought from Luna in Edron; and Holy Water; bought from Henricus in Thais for 1,000 gold.

When you have obtained all three of these items, use the Star Herb on the Antlers to obtain Antler Talisman. Then use Holy Water on the Antler Talisman to obtain Sacred Antler Talisman.

With the Sacred Antler Talisman, head to the Tainted Souls Cave north of the Green Claw Swamp. In this cave you will encounter Tainted Souls, Toads, Gloom Wolves, and Witches. The cave has 4 floors with the same monsters on each floor. Explore the caves and use the Sacred Antler Talisman on all the Tainted Souls that you find. Do this until you have freed 100 souls.

When you have freed 100 souls, go to the 4th floor and enter the Mystic Flame. You will be teleported to a nearby room with Ekatrix. Kill her.