There are hardly any extraplanar beings in the 1e Fiend Folio: a few ethereal and astral foes, one devil, the demoness Lolth, the slaadi and the Elemental Evils. This suggests a much simpler cosmology than the standard "Wheel of the Planes." Consider a Prime Materium at the center of the four nested Elemental Planes, arranged in a shifting order depending on their current influence on the Prime. Astro-ethereal space interpenetrates all of the planes, and beyond the outermost Elemental Plane lies only the chaos of Limbo. Crucible, the Prime, is the warring ground of the Lords of Elemental Evil.
Imix, the Firelord, is currently ascendant; smoke seems to drift across the night sky, a haze beyond the embral red stars. Hordes of firenewts, led into battle by suicidal berserkers in magically heated armor, have driven the dwarves down from their mountain keeps. Firetoads and drakes harass the elvish woodlands and the halfling and human farmlands. Mephits plague the cities: smokish mephits choke and wheeze the unwary...the magmic delight in vandalism and desecration, melting statuary and gravestones...every woodpile and thatched roof is kindling for the fiery mephits...the steamish poison wells.
The other Elemental Lords have less influence. The icy wastes and cold seas that gird Crucible's equator are still held by the hostile allies Cryonax the Icelord and Olhydra of the Wave. Fog giants, frost men and ice drakes maraud the tundra, and kuo-toa return to the oceans from their caverns and earthen deeps. The Earthlord and the Airlord are weaker still, but they too contest for the Prime. It is thought that the slaadi manipulate all five of the Elemental lords, to unknown ends.
The clerics of Crucible, no matter the ethos, seek vainly for gods to worship. Undead are few, so they channel their nameless faith to turn or control the Elementals.