I wanted to know the reactions that the law enforcement in Sigil would take against a law breaking fiend in Sigil. Now obviously if a Balor or Gelugon started massacring people in the street, the Lady would probably maze or flay them before too long but what about if a fiend started a disturbance at a bar? Or what if the bar room brawl happened at a lower planar bar in Sigil? How about if someone got killed during said brawl. Would the Harmonium get involved? I imagine that dealing with fiends would be beyond the scope of a typical Harmonium patrol, do they have special anti-fiend forces in the reserves? Or if said fiend is untouchable by Harmonium and flees, I'm guessing the mercykillers would assign one of their own to go after them.
Mostly it would depend on the place, time and the rank of the fiend in quesion. It is unlikely that a low ranking fiend will find much back up from their higher ups. In the event of a capture, baatezu will probably go to court to free the fiend, than sue the people who put them there for the trouble. Tanar'ri will probably try to free it by bribery, threat or force, (but theyprobably won't even care).
Harmonium(pre- Faction War) was most effective in the Lady's ward, and the areas surrounding it whereas they were almost non existant in hive and Lower. It is fun to think they had some special task forces for extreme situations. Mercykillers too, or perhaps even joint forces of both could be employed on dire occasions.
Almost everyone has a weapon or some means of defense, or they have escorts, guards, familiars etc. and every bar has some bouncers to keep the place cool. Who knows what happens in a back alley in Hive or a shady basement tavern in Lower Ward?
Decent people go to bed before antipeak anyway.