Instructions
Complete the Dead City Watch hunt to receive a Soul of Thamasa. If you are working your way through the hunts in the game, this is the best way to acquire a Soul of Thamasa. Visit a tavern and look at the hunt board. Agree to the Dead City Watch hunt and then talk the petitioner Popol at the Jajim Bazaar in the Nalbina Fortress. Visit the Necrohol of Nabudis and make your way to the Cloister of the Highborn. Deathscythe (the monster you need to defeat) will only appear if two of your characters are in critical state (low HP). Defeat Deathscythe and visit the guild to receive your prize.
Defeat Oversouls in the Necrohol of Nabudis until one drops a Soul of Thamasa. Oversouls are standard monsters that roam throughout the Necrohol of Nabudis. Purchasing the Warmage's Monograph Grimoire from the Bazaar greatly increases the chance that an Oversoul will drop a Soul of Thamasa.
Steal a Soul of Thamasa from Ishteen in the Barheim Passage. Ishteen is rare game, and it will only appear in the East-West Bypass and Zeviah Span areas in the Barheim Passage. Once you are in one of these areas, wait for the Ishteen to appear. This may take several minutes.