I had pest control over (I don't want the 2 year old wandering nearby) and what luck, some animal came along and dug out the nest between my mowing and his arrival. My guess is the Raid I used to knock down the swarm enticed some critter in to clean up the dead ones. What workers were left buzzing around looked like bald-faced hornets to him, but that surprised me given I saw no documentation of nesting underground. Either way they hurt, and I'm glad they're gone.
I've been looking to get some for the yard myself, so I don't know much besides what I've researched. Any signs of fungus or pests?