Xaecreotis, the Realms Below

Believed to somehow be the foundation of most other planes, Xaecreotis is adequately dubbed "The Realms Below." An underground world, Xaecreotis simultaneously defies and maintains many of the rules that govern caverns and caves in planes like Emaxus and Gliziarial. Xaecreotis has one of, if not the most numerous collections of planar connections out of all the planes of Yophas, and its biomes and inhabitants reflect that. Records of the Realms Below show that it is divided into several, continent-sized caverns and their countless smaller corridors and caves. Within these continents, there are underground oceans, fungal forests, subterranean empires, ancient entities, and much, much more. In locales where planar connections reside -- at least those connections that are more permanent -- the geography is often morphed by whatever plane is on the other side. Some regions have taken on hellish heat and magma slides due to rifts to Iduran, while others have grown infested and overtaken by insects and other swarming creatures due to portals to Ix'akroth.
Essentially, Xaecreotis is just as varied as Emaxus itself, only everything is underground, usually lightless -- save for some bioluminescence or magic -- and very inhospitable to non-natives. Even natives of Xaecreotis can find it difficult to survive, as the Realms Below are very much a "kill or be killed" territory.

Inhabitants

Xaecreotis has incredibly diverse native species, amplified by planar travelers, settlers, and even invaders. Drow, deep dwarves, deep gnomes, and fire giants form the backbone of mortal societies in Xaecreotis, while fauna ranging from lizard-like humanoids to titanic worms and underground dragons form intricate predator-prey relationships. Beyond that, illithid and other aliens from Ipacia and Xeccamund find their way to Xaecreotis, as do infernal insurgents from Ix'akroth, Accorion, and Iduran. With such varied -- and often violent -- inhabitants, Xaecreotis is a violent, warring plane. Everything from small insects to Xaecreotian empires seek to annihilate one another, and the very plane is scarred by conflict.

Travel

Travel to Xaecreotis is easier than to most Divine Planes. Xaecreotis has semi-permanent and/or numerous rifts to Iduran, Ix'akroth, Mathrak, Accorion, and even Xeccamund. Alongside those planar connections, many underground cavern systems in Emaxus find their way into Xaecreotis, by some means or another, as do deep caves in Gliziarial and Vokatha, and that's on top of normal planar magic or Ipacian navigation.
Travel within Xaecreotis is one of the most difficult things planar travelers can attempt. Beyond even the extreme hostility of the flora and fauna of Xaecreotis, the very geography of the Realms Below is alien to the way most other planes work. Shifting tectonics and collapsing caves can lead to entire regions moving from year to year, and ever-changing political boundaries can create bureaucratic hassles -- if the powers that be don't kill on sight, that is.

Type

Dimensional plane