After a successful adventure and money in the pocket they decide to splurge and live it up. The Silver’s Nest comes highly recommended and it quite often frequented by the higher nobility of the city. They could have similarly been invited there by one of the nobility to discuss some employment opportunity. Either way they site down and order a nice meal and it far surpasses their expectations until halfway through their meal other guests start falling over choking and vomiting.
Priests are called but for many it is already too late.
Assassination attempt. Can anybody be that sloppy or heartless? Perhaps it is somebody trying to get a guilds attention? An ignorant kitchen hand bribed by a ruthless noble looking to get rid of some rivals. Were the players themselves the targets? There a conflict brewing between nations? Preemptive strike by the other nations? Poisoning may have happened all over the city.
Accident? Ignorant kitchen hand picking the incorrect herbs. New fish that was added to the dish that was prepared correctly during the test phase and when tried on customers it wasn’t prepared correctly.