Mortal Causes by Ian Rankin (Inspector Rebus #6)

Underneath the cobbled streets of Edinburgh's old Town are medieval stone cellars where a man could scream and never be heard. In mortal Causes, the tortured body of a young man is found hanging from a butcher's hook in one of these underground rooms. The tattoo on his wrist and a cryptic inscription scratched in the dirt suggest to Inspector John Rebus that this was an execution, but what man or men carried it out?