You'll need to gain access to Crow's Nest by completing the quest Bark up the Right Tree first. You'll find Decimus' House east of Henry and Eleanor's House and southeast of Crowell's House. Inside you'll encounter the bandit Carmina who is a target of the quest A Hunting We Shall Go Once More. After defeating Carmina and her thugs, take Livia's Ring from one of the small lockboxes. You can either give the ring to Decimus to complete this quest, or give it to Astridax for the quest Sinister Motives instead.