A large explosion brought us all out onto the street where a group of gnomish researchers (mad scientists?) were looking a little sheepish, and a little shellshocked. It seems that one of the artifacts we brought back from the Elemental Caves was the subject of their study and experimentation. They were trying to scan it with various types of magical energy imaging techniques. And it got tired of being poked and prodded, and decided to strike back. A hole in the ground showed the path it took as it burrowed its way down into the earth, and then with a 90° turn it headed out of town.
From the direction it headed, a set of booming crashes were now audible, in a particular cadence. It was the rhythm of a mechanized colossus, one that looked like a railroad steam train crossed with an Imperial Walker. Considering the artifact which summoned this infernal engine was an earth/fire hybrid, this makes sense. Despite its appearance as a blue glowing gem, it might be better understood as the spirit of vulcanism incarnate.
This thing had 4 legs with articulating joints at ankles, knees and shoulders. It had a large tube out of which a fire elemental could be shot as the dorsal ridge of its torso, and it had a cow-catcher as its chin beard, to make its head truly intimidating.
Tenley and Necklehurst tackled the elementals, while Whisk flew up onto its back and started to attach the torso. We made no headway at first, but after enough magical damage was done, we eventually got its tightly buttoned up armor plating to relax a little and then the rapier Whisk aimed at the cracks and gaps was able to get in between them and do damage. The unstoppable Necklehurst and the indomitable Tenley, both wielding hammers but the former more defensively and the latter more in a damage-dealing rain of blows, were stupendously creative and brave in their persistent attacks of this monstrous steampunk nightmare. Eventually we damaged enough of the torso and head to stop the colussus from advancing towards Dirtwater.
We had to fight for a number of rounds after that until all the legs were severed from the torso, and finally we killed it, and took the blue gem out of the pilot's seat, capturing it to take back to the gnomes, perhaps?
A really lucky roll on loot table-- 12,12,12,10 for 4D12--- yielded an Enlightening Relic. We also got a recipe for a health potion. Homing Compasses, and Charging Arrows or something that sounded like that.