One of the most important elements of world-building for my money is the Underworld. Rich treasure troves of traditional material await the mythopoet here: demonic hordes, devilish hierarchies, hungry shades, imprisoned spirits -- on and on the list goes. All this can be mined, no matter where you fall on the re-usage continuum from pastiche to alchemical transformation. For today's mythopoeic experiment, I will draw on recent mythological elements brought up on this blog, so Norse and Judeo-Christian elements will predominate. To populate them, I will confine myself to creatures from the Moldvay and Cook D&D books. This will give us some more perimeters to structure our creativity.
Since for the purposes of this experiment I am drawing on gaming materials, we'll assume that this is an Underworld for an RPG campaign. Starting with the Underworld is especially a good beginning for a fantasy RPG world because of the traditional importance of the dungeon, which is where the World and the Underworld start to blend together as one slowly descends into the Underworld. (For more about this, see the resources linked here.) We'll assume a traditional three-tiered universe of Overworld, World, and Underworld. Since we already have the World Tree (Yggdrasil) in play, the three worlds will be joined together by this cosmic tree, which will allow for heroes and monsters to travel between planar locations.
With part II, we'll jump right into building the Underworld by rummaging around in our inspiration hoard.