Setting: partly mountainous, partly firey underworld, partly airborne.
Hati, the Sky Wolf, has caught up with Sol’s* chariot, and devoured her brightness to fling Midgard into eternal night, where Mani (the moon goddess) is pursued ever more closely by the other hideous Sky Wolf, Skoell.
The characters must travel to Muspells-heim, the Nordic underworld, to find Wanrefax (DarkMane) the Black Dragon, subdue him, and ride him to joust with the Sky Wolves, releasing the sun from the cavernous belly of Hati.
This adventure is based in a Nordic paradigm (as you might have guessed from the Norse Mythological references), but could easily be adapted for other world-settings. It is a high level adventure though (bear in mind that these wolves are not only big enough to eat the sun and/or moon, but have sufficiently strong gastric juices to digest them!).
*Sol - the Norse goddess of the sun.