Shadrack was a stuntman at a circus, who could insert his head into his lion's open mouth without getting his head eaten. However, during one of these stunts, the lion apparently bit Shadrack's head off, killing him in October of 1898.
Following his death, Shadrack's spirit remained on earth and unable to move on, due to unresolved issues. During Dr. Zorba's travels around the world, he met and captured Shadrack and imprisoned him in his mansion, along with the ghost of the lion that killed him during his act in New York City.
When Dr. Zorba was murdered and became the twelfth ghost, Cyrus Zorba and his family moved into the mansion. One day, when Buck explores the cellar and finds Shadrack's top hat, the ghosts of the headless Shadrack and the lion that killed him appear, with Shadrack looking into the lion's mouth for his head to be back on his body.
Shadrack and the other eleven ghosts presumably crossed over when Benjamin Rush died and became the thirteenth ghost.