This isn't so much a bug, as just an observation.  Did the Boots of the Spider event.  Finished the big battle, got the boots, and then was greeted with the following (the full red circle is one group of 6 that I killed before I thought to take a screenshot):

I have no problem with this actually.  The unintended consequences of the quest were great and I had a blast pulling out every magical trick I had to save Hianan.  But I thought I'd point it out, just to be sure that this many groups is what was intended.  Hopefully it was intended.  

Each army was 4 to 6 groups of swarm spiders.  200 to 300 total hit points per army.  Took two armies of two heroes each a few turns and probably 400 or so mana to save that city.  The city got attacked a few times as well, but I managed to weaken the groups on the strategic map enough that the city defenders were able to withstand the assault.  Good times.