The Crusade against darkness was fought for many years, crushing mercilessly any that they deemed evil. Monster after another was destroyed, or driven away to the wastelands, or the great darkness. They sought to eradicate all evil, and bring peace upon this land. Long time has passed since then, and the people have grown proud and lazy. They think they have defeated the evil in these lands, and that evil has been forever broken, banished away.
Within the stone and dirt of the great darkness, a heart began to slowly beat. Crimson eyes opened, and a young man slowly pulled himself to his feet. The chamber's floor was hard stone inlaid with mystical sigils around a pool with crimson glowing water. Within the water, a heart the size of a bed was beating slowly, but strongly. The walls were pushing back slowly, dirt pushed back and shattering, stone cracking and giving way leaving a smooth stone surface behind, a coating of stone replacing the last dirt walls, seeming to strengthen, as the walls of that chamber turned stronger, and stronger. Mana was infusing the chamber, and the heart's crimson glow illuminated the room even through the water within the round pool.
One of the walls cracked open large enough for a passage, and as it did, shadowy figures appeared there, given shape by the heart. He could feel the connection to the heart, and these shadows. They obeyed him. They were little more than 'dolls' that obeyed his will, digging their way through both dirt and weak stone quickly, starting to make way for passages, rooms, and to reach out to the distance, seeking riches. Seeking gold. Seeking... the weak spots in the stronger stone, a pathway to the world above where the monsters had been banished from, and the passage to the deeper lairs of banished monsters.
He knew instinctively... that in order for the world to exist, it needed evil as much as it needed good. Corruption awaited if either side got too strong.
The game would take after 'Dungeon Keeper' series in a sense, partially being about an extending 'dungeon' the characters would both inhabit and defend, and the world above and below. The great darkness where monsters have been finding shelter from, and the above where they once dwelled as well before the great crusade.
It would also give room for the 'Master' to walk among the ones that would join him though, taking physical form, though preferring to still build his strength and protect the secret 'Dungeon Heart'. The heart a powerful magical place that both serves to 'bring back to life' at least the Master, but likely any fallen character as well.. trying to defend their home, and playing it from the side that 'monsters are not all sadistic bastards'. Perhaps partially being that at times, but also having their own reasons.
Would probably be a free-form game for those wanting to play monsters.
Would have NPC heroes try and breach into the tunnels at times, the monsters roaming the passages finding new passages as the dark 'minions' sought more wealth, and strengthened that 'stronghold' underground.
Could be there were also other 'Dungeon Masters' rising up in the darkness too, but some of them might be more sadistic, too sadistic even.. some might prefer one race over all others... (like a 'DM that only wants girls' for example) and so... there would be conflict... but there would be a theme as well.
Evil will always find a way...