His name isn't Jack, but that's what the newspapers have dubbed him, and he answers to it readily enough. The police, led by their pet miracle worker, have been chasing him through London's night alleys. But their miracle worker isn't half the genius Holmes is, and he and John reached their quarry first.
He is a monster. Just as the Great Ones who rule their world are evil, his eyes are filled with slaughter.
"He'll do," says Holmes.
John listens silently, fearing and hoping, as Holmes pours words into Jack's ears, tempting him with the murder of their eternal Queen.