Throughout the Prison de Joaquin, you will find long, metallic sarcophagi, which are clickable. Clicking on them however does nothing. And standing close to one usually result in an exclaimation (!) from the sarcophagus and your characters start taking damage from no apparent source.
In fact, the damage is dealt by invisible spirits of Joaquin, located within the sarcophagus. Simply stand close to the sarcophagus and use Detection skill. The Spirits of Joaquin will appear and attack. These spirits seem to have a higher drop rate than regular mobs in the prison. (The monster level varies, depending on which level of the prison you are at. See stat blocks below.)