Megadungeon Concept: The Black City

I had a few competing ideas for a megadungeon setting, quizzed my players and also ran a simple poll on Dragonsfoot to help choose from amongst three unrelated ideas.  What follows is the concept that proved most popular, and which I'll be developing here on the blog.

The Black City
The dragon-necked warships of the northmen raiders sail far and wide in search of treasure. Across the misty northern sea, at the very ends of the world, the northmen discovered a rocky land and an ancient ruin. Song and legend called this the land of the Hyperboreans, those ancient men that first discovered magic when the world was new. Now the 'viking men' have established a trading camp on that distant shore, and bands of rugged fighters plumb the depths of the ruined city seeking loot and glory.

The ruins are a sprawling expanse of basalt buildings and strange, cyclopean architecture, very much inspired by the Antarctic city of the Elder Things in 'At the Mountains of Madness'. Whether it was built by the Hyperboreans or some non-human race, the city was once inhabited by a prehistoric and decadent race, the first users of magic. The ruins will feature elements of weird science and alien magic. What doom befell the city?  Perhaps the city was built in a time before the world shifted on its axis, plunging a verdant land into the cold northern wastes, or perhaps the slave races revolted and overthrew their master in ages past - much like the demise of the Elder Things.

The Northmen will have a trading camp at the harbor not far from the ruined city. Armed groups of adventurers can use the camp as a base for forays into the ruins. Not far from the entrance to the city will be the main egress point into the sprawling dungeons beneath the city; most adventuring parties follow that route. The dungeons get warmer the deeper you go, lingering effects of an ancient technology. Experienced groups could explore deeper into the frozen, ruined city itelf, and face tougher surface monsters equipped for the cold that still haunt and hunt in the ruins.

The dungeons will be the ancient laboratories and halls where the ancient scientist-wizards perfected their arts; there will be no lack of strangeness down there.  The monsters too will have to be stranger and more alien than a traditional D&D game... I see it using Pulp Horror as the primary genre - we came, we saw, we kicked their ass. And then we ran because the elder god showed up.

The fantasy northmen will have a culture similar to the real world vikings, but the rest of the world will draw more inspiration from the Hyborian Age than Dark Ages Europe.  I'm thinking of mining the old "Hollow World" boxed set for some ideas on cultures and game stats, since I never actually used it as a hollow world setting back in the day, but it's chock full of good ideas.

Note:  I was originally calling this The Lost City of the Hyperborians, then I saw what these guys are working on... yeah, I realize Hyperborea is pretty common in CAS and HPL and REH,  but I'll probably opt for another name nonetheless.