In D&D terms it will be one of those spooky old castle ruins on the hill where a crazed wizard conducted diabolic experiments, unleashed unspeakable horrors from the realms of pure chaos, and that have been over-run with hordes of filthy humanoids. And all with plenty of gold to keep the desperate and greedy coming back for more. And that's just the ground floor - you should see what the basement holds!
Either way, it's going to be a sprawling and flexible system that allows for furniture, doorways, traps, multi-level rooms, and the potential for slaughterhouse levels of rended flesh. Hence the name. For figures, I've got cultists, pig-faced orcs, frogmen, undead, and a handful of the sorts of strange and bizarre creatures one is likely to meet in the Mythic Underworld. I may even be able to tap into my 10mm collection to throw some surprises at the wary and unwary alike.
Now I've looked at a few different systems, and after struggling through the Big Box Blues, decided to build 2.5d Dungeon Tiles as popularized by the CraftyDM (YouTube Channel) and DMscotty. As with all things wargamey, we'll put our own unique spin on it, though.