It’s not fanfic either 
You know, a good caper can often do with a good complication–an “oh crap” moment when everything goes wrong, and the rogues have to scramble and improvise. This thread was apparently on my mind overnight, because I dreamed up a doozy of a complication for it.
In the ancient past, before the Aesir, the Jotunn had another enemy: ancient beings that preyed upon them, hunting them for food, or sport, or some alien whim. In time, the Jotunn managed to unite and fight back against them, but could not destroy them. All the Jotunn could do was drive them back and imprison them in ice at the ends of the world. The Jotunn then did their best to wipe all trace of the ancients from the world, burying the memory of long ages of fear and shame beneath new myths. Few even among the Jotunn remember the truth.
The carvings on the Jarknasteinn include words in a very old dialect that speak of an ancient city of vast wealth, hidden beneath a glacier, and promise that the stone is the key to the city. This is a half-truth–it is a key, but the key to the prison of ice that binds the ancient horrors. The creatures are trapped, but they were able to slowly, ever so slowly, etch a message into a special sunstone, charged with power. If held aloft in the right place, at the right time, it will channel the power of the sun to melt their prison, and a new age of horror will ride the resulting flood throughout the worlds.
Obviously, your rogues will have to learn all of this when it appears too late to stop it, resulting in a mad chase and a dramatic climax on a crumbling glacier.
Balance, that’s the coolest idea I’ve read. I’ll definitely use it. Thanks. Combined mythologies might be nice; for example Niflheim being pretty much the same as Pohjola in Finnish myth
(Also you win this thread.)
I’m a sucker for mixing Lovecraftian elements into stuff, and Scandinavian myths seem as good a place as any. 
This notion could be made to dovetail with Kobal2’s suggestion of using the stone to navigate the waters of other worlds. Perhaps the ancient city can only be reached by locating the mouth of the Ifing (the river that separates Jotunheim from Asgard), and following it to its headwaters. That’s dipping into cosmological adaptations, of course–not sure how deep you want to go there.
Cosmological adaptation would actually be a great idea. Lovecraft’s not the only writer to write about ancient evil creatures; JRRT mentions “dark things” in The Hobbit and LOTR.
That said, “mysterious alien race living under the ice” makes me think of the Lagarfljót Worm. Imagine a whole tribe of those.
Granted, Sealed Evil in a Can comes in many flavors. I usually refer to the Ancient Unfathomable Horror flavor as “Lovecraftian”, even if it doesn’t actually contain any Mythos chips.
Like a secret nest of ice-dwelling dragons.
On a related topic, anyone know of any good non-fiction books on the Lapland or “Finnish”/Saami witches and their magic?