(Please note that apart from the Honeyngton article and Rats under Honeyngton Part 1 most of the articles aren't designed to be used in a particular order, though this scenario works best if the heroes have not yet found out about any cultists in Honeyngton and ideally the connection should only become apparent later when the realise the temple of Travia was the intended target)
Lost fireworks
A traveling circus is in town (green pentagon on the map for their campsite) and one of their attractions is a fire eating Dwarf and for their last night in town they announced a big firework display (outside the city walls to the west to prevent fire to the city)
A few days before that event, the chest with the already prepared fireworks are stolen by some of the children of the rat cultists with the aim of creating a firestorm in the city. Their target is to set it off between city hall and the temple of Travia (red X on the map) in hopes of destroying or at least damaging both buildings and as many other buildings in the neighbourhood.
Jandrim will warn the heroes to keep the fireworks in their chest, which is clad with plates of Kosh-Basalt, to prevent it to attract fire spirits. Kosh-Basalt is a mineral with anti magical properties. This makes the chest rather heavy and the cultists didn't get that advice.
One of the cultists is too curious for her own good and took one piece of fireworks to her home in the southwestern quarter of Honeyngton (see red shaded area on the map). This charge can go off as a dramatic device whenever needed to enhance the plot.
This scenario allows for some tracking (the thieves used a small cart during one of the shows to carry the heavy chest with fireworks away) searching ( where are the fireworks stored? The sewers are too moist, the deserted castle construction site would be better, see red O on the map for a suggested location) hunting (if the heroes come to late to the hideout and now need to race the cultists) and firefighting (if the heroes come too late to stop the fireworks, they can at least organise and coordinate to prevent spreading)
In this scenario the cultists involved would have little knowledge of the cult, being of the unknowing and should appear just as some random criminals. Their main target should appear to be city hall and the temple seems to be collateral damage due to the location they were instructed to set the fireworks off from.
Map with relevant places marked:
