It is the midsummer of the 8th year since the founding of Plainschannel, and the overseer is playing clue... with the dwarfs.
Coincidentally, two plagues are simultaneously sweeping the fortress. One causes severe and extended vomiting, while the other causes rot and slow, painful death. Ironically, the progression of the plagues is the perfect excuse for isolating dwarfs to narrow down the culprit list. Infected dwarfs are quarantined in their rooms, and eliminated from the suspect list on the event of an attack. As a precaution, the meeting hall has been flooded with slimers and jackalopes to act as a buffer to prevent violence between dwarfs and thus the loyalty cascade that follows. Upon the event of an attack on a pet or fellow dwarf, the attacker is eliminated from the suspect list.
Summer rolls by. The source of the rotting plague has been discovered. It seems that a nephal merchant arrived with the disease, transferring it to hapless dwarfs as they loaded the trade depot with golden crafts and riches.
In the first month of autumn, and only three suspects remain: Atir the carpenter, Sakzul the planter, and Fikod the mason. The break comes as Olon the metal-smith suddenly strikes a slimer. Sakzul, the only other dwarf in the room, does not flee and thus confirms his guilt as the long-sought traitor. Immediate isolation of Sakzul ended the attacks immediately. Thus one problem is solved, while another is posed: What to do with Sakzul?"
Lessons and facts learned:
1. Traitors do not immediately take action. Sakzul waited two years before attempting sabotage.
2. Traitors can be one of the original seven. Sakzul is one of the original seven, and I find this very surprising.
3. Flooding the meeting area with pets (preferably non-lethal, and definitely not zingers!) works very well for preventing turned dwarfs from causing a loyalty cascade. The turned dwarf simply beats up a helpless pet and shortly finishes before laying a hand on any other dwarfs.
4. Plagues can come from merchants. I'm not 100% sure about this, but it seemed that the nephal merchant did in fact bring the rotting plague, as there was a merchant with symptoms and isolation from the depot prevent the initial spread.