Ruinous Idols appear as a red shadow in the first half of the Harabah Block within Tartarus. They will be the subject of one of Elizabeth's Requests, and will drop Moon Tablets while the quest is active. Returning to Elizabeth with 3 Moon Tablets will complete the quest. They will later appear in the second half of the Harabah Block, from floors 191 to 200, where they may encountered in groups as large as four.
In The Answer, Ruinous Idols will appear in the second and third sections of Caina within the Abyss of Time.