The quest "The Return of the Fallen Gods" is available from level 14 and can be obtained from the Forest and Lake hermits on the Isles of Swirling Mist. They will send you to the ancient altars on the Daigon Islands and Scolopendra Coast.
The Spirit from the Shaat-has Altar will teach you how to kill the desecrated monsters and shatter the Shiass Shard of them. For each killing of the Nepherto Shevass , who lives in the Grotto of Suffering, you get the Small Shiass Fragment provided you have studied the Third Shiass Thesaurus.
After studying the Fourth Shiass Thesaurus, you will receive the Little Shiass Fragment, after each killing of the Cerebhound Mohgar  who lives in the Mouth of Darkness.
For each killing of the Droolizard Gershess , who lives in the Death Mine, you get the Medium Shiass Fragment provided you have studied the Fifth Shiass Thesaurus.
For each killing of the Kretch Argash , who lives in the Hall of Horrors, you get the Large Shiass Fragment provided you have studied the Sixth Shiass Thesaurus.
For any further defeat of these monsters, you will continue to receive Shiass Shards, to increase your reputation as a Fighter against Shiass.