A Bed of Earth — Tanith Lee

In this “deliciously creepy” novel by the Bram Stoker Award winner, two feuding families face supernatural vengeance in a parallel 16th-century Venice (Publishers Weekly).
 
In the City of Venus, two noble families -- the della Scorpias and the Barbarons -- have been locked in a bitter dispute over burial grounds on the overcrowded Isle of the Dead. But it is fourteen-year-old Meralda della Scorpia who pays the ultimate price for their rivalry.
 
As years pass, parties complicit in her disappearance begin to suffer the consequences. Their shocking deaths can only mean one thing: A supernatural force has been unearthed from the city's rotting understructure. As these bizarre events throw the city into a panic, a humble apprentice gravedigger is left to sort out the mysteries and subdue the ancient terror that threatens to destroy the entire republic.