From "The Frost Monstreme"
Inside a sphere half again as tall as a man, a skinny old being was busy. On the interior of the sphere was depicted a world map of Nehwon, the seas in blackest blues, the lands in blackest greens and browns, yet all darkly agleam like blued, greened, and browned iron, creating the illusion that the sphere was a giant bubble rising forever through infinite murky, oily waters ― as some Lankhmar philosophers assert is veriest truth about Nehwon-world itself. South of the Eastern Lands in the Great Equatorial Ocean there was even depicted a ring-shaped water wall a span across and three fingers high, such as those same philosophers say hides the sun from the half of Nehwon it is floating across, though no blinding solar disk now lay in the bottom of the liquid crater, but only a pale glow sufficient to light the sphere's interior.
Where they were not hid by a loose, light robe, the old being's four long, ever-active limbs were covered by short, stiff black hairs either grizzled or filmed with ice, while Its narrow face was nasty as a spider's. Now It lifted Its leathery lips and nervously questing long-nailed fingers toward an area of the map where a tiny, gleaming black blotch south of blue and amidst brown signified Lankhmar City on the southron coast of the Inner Sea. Was it Its breath that showed frosty, or did Its will conjure up the white wisp that streaked across the black blotch? Whichever, the vapor vanished.
It muttered high-pitched in Mingolish, “They're gone, the bitches. Khahkht sees each fly die, and sends Its shriveling breath where'er It will. Mingols harry, world unwary. Harlots fumble, heroes stumble. And now 'tis time, 'tis time, 'tis time to gin to build the frost monstreme.”
It opened a circular trap door in the South Polar Regions and lowered Itself out on a thin line.
The red flare died. Pshawri came down rather jauntily from the top and reported to his dread captain, who dismissed him with a glare which was unexpectedly terminated by a broad wink and the command to burn another flare at the next bell, or demi-hour. Then turning once more to Ourph, the Mouser spoke low: “Talking of wizards, do you know of Khahkht?”
The ancient let five heartbeats go by, then croaked, “Khahkht is Khahkht. It is no tribal sorcerer, 'tis sure. It dwells in farthest north within a dome ― some say a floating globe ― of blackest ice, from whence It watches the least deeds of men, devising evil every chance It gets, as when the stars are right ― better say wrong ― and all the Gods asleep. Mingols dread Khahkht and yet ... whene'er they reach a grand climacteric they turn to It, beseech It ride ahead before their greatest, bloodiest centaurings. Ice is Its favored quarter, ice Its tool, and icy breath Its surest sign save blink.”
“Blink?” the Mouser asked uneasily.
“Sunlight or moonlight shining back from ice,” the Mingol replied. “Ice blink.”
Inside the dark-mapped globular vacuity, It ceased Its dartings, held Itself rigidly erect, facing away from the water-walled equatoriaql sun disk, and intoned in voice like grinding ice flowes, “Heed me, smallest atomies, that in rime seas seethe and freeze. Hear me, spirits of the cold, then do straigtway what you're told. Ships are meeting, heroes greeting; gift to each, from each, of death. Monstreme lurk, in icy murk, picket of the Mingol work 'gainst each city, hearth, and kirk. If they 'scape the Viewless's ruse, make yourself of direst use. Vessels shatter! Man-bones scatter! Bloody flesh, bones darkness splatter! ― every splinter, every tatter! Deeds of darkness, darkness merit ― so, till's done, put out the sun!”
And with reptilian swiftness It whipped around and clapped a blacked-iron lid over the softly flaring, walled solar disk, which plunged the spherical cavity into an absolute blackness, wherein It whispered grindingly and chucklesome, “...and the Ghouls conjured the sun out of Heaven, quotha! Ghouls, indeed! ― ever o'er-boastful. Khahkht never boasts, but does!”
Personally, Leiber is my favorite author. His tales of Fafhrd and the Mouser have had more influence on my thinking about fantasy adventure gaming than any other source.
What I've quoted above, snippets concerning the Ice Wizard, Khahkht, would be an excellent fit in Hyperborea. An unseen hand, perhaps. Likewise, Ningauble's cave must certainly have a twisting passage which eventually transcends the Black Gulf, and could provide passage for adventurers from innumerable worlds and realities.....