The Singing Bone

Most small towns of Xotera have a variation of this story told with different flourishes or details, but always including key details of the moons, the stranger, and the instrument.
A young enough looking man made his way down a darkened street at night under three new moons⁠—the darkest of nights that only occurred when Zathandra, Meteorcrite, and Xendathu aligned in their cycles to cast no light on Xotera. Without the aid of moonlight, he failed to see an assailant waiting around a corner, and the man was murdered, his body left on the street. The killer destroyed the corpse's face to the point that not even the town's most skilled holy man could speak with him. The next day, the townspeople gathered to lament the slaying and the brutal way his body was defaced, but nothing was to be done. Soon the spectacle faded and the man was buried without fanfare. 
Years later, under the same three new moons, a stranger came to the town, shrouded in dark cloaks and a mask, but he claimed to be able to speak with the bones of the dead. He said he was called to this place because a murder had not been resolved and the spirit of the deceased did not allow him rest. The stranger requested the longest bone in the man's body, which was fetched for him after furious debate about the propriety of disturbing the dead. For two days, the man hunched over the bone, muttering incessantly and chipping away at the bone here and there.
Finally, his task done, he brandished a rudimentary flute whittled from the femur. A somewhat horrified crowd gathered as he began to play a somber tune. A few bars into the song, a child nearby began to sing a story as though compelled, accusing the town's butcher of the murder with such detail that none dared doubt it. 
His job done, the stranger tucked the flute into his satchel and walked down the path a ways before simply vanishing.