You begin by secretly selecting 3 actions you would like to take (similar to race for the galaxy) with the goal being to gain resources. Resources include food to pay monsters with and buy imps; gold to pay taxes and buy traps/food; imps to work in rooms, mine tunnels and excavate more tunnels; traps and monsters to fight off heroes, and rooms to produce resources and gain victory points.
You are required to pay taxes, pay your monsters, and have a special (randomly drawn?) event happen each of the two years and these events happen in random order - though you're given some warning so you can plan. After these 3 things happen, you're invaded by the heroes who've decided to raid your dungeon over the course of the year with the hardiest heroes targeting the most evil dungeon lords.
As you're an evil dungeon lord, hopefully you've accumulated enough traps and monsters to counter the threat these do-gooders present. Thieves defend the party against traps, warriors are tough and are the first in line to be attacked, wizards cast nasty spells and Priests heal the adventurers in front. As a dungeon lord, it's a lot to deal with.
After 2 years of being attacked (of 3-4 turns each), a good portion of your dungeon is likely to have been conquered by these nefarious beacons of light and justice. These regions count for negative points, while unconquered regions will score you points. Titles are granted to the most evil dungeon master, the one with the most rooms, tunnels, etc. These are worth big points in the context of the game and can undo a lot of damage done by having conquered tiles.
Overall the game is blast, but I find it only worthy of a B+ or A- rating overall due to the complexity of the rules. It's no Battlestar Galactica or Arkham horror in that way, but a lot of quirky little exceptions add up in the initial rule reading of an already complex game. That said, once you're through the rule reading, it's as good a game as you'll find.
Games it's like: Kingsburg(3 seasons and attack, limited role selection), Agricola(Pacing of harvests/worker placement), Race for the Galaxy (secret role selection), Galaxy trucker (watch your creation destroyed by malevolent forces), Vegas Showdown (tile placement)