Elysium

Elysium is one of the Throne Worlds - universes existing on a radically higher energy level than the much more common Vassal Worlds. As a result of this, Elysium simply doesn't play according to rules recognizable to inhabitants of more 'natural' planes of existence. In harsher words, it would be nonsensical - if it didn't follow its own rules.
In the far past, it was the homeworld - and seat of power - of Metatron, one of the twelve Dark Lords. Simaar was deposed following the Great Multiversal War, however her influence still permeats the entire plane, with every single detail of this universe once sculpted by her insane hands.

Geography

Elysium is a three dimensional world, composed mostly of light-permeated nothingness. The only habitable parts are the numerous floating islands, typically few square miles in size. Most of them are pleasant, if not idyllic landscapes. Some, instead, are covered in cities and fortresses of gold and silver, ancient rampants either constructed by the Dark Lord or predating him - today these places are inhabited by the celestials.
In the center of this realm lies the Panopticon, once a palace, throneroom and a fortress at once for Metatron herself. What exactly Mankind did to the local Dark Lord during the Great Multiversal War is anyone's guess (only the Dominion leadership knows, and it doesn't say a lot), but the Panopticon is now sequestered in a pocket dimension to avoid its inhabitants from spilling outward and threatening Elysium as a whole. Uncensored bits of reports and memoirs from the time describe it as a 'throne city of gold and jewels', from which the Metatron observed her entire Realm (and her domains in Vassal Worlds) at once.
The islands of Elysium continue on endlessly - in theory. Beyond the regularly inhabited celestial territories lies the Expanse - the place where Reality is out for lunch even more than usually among the Throne Worlds. Within the Expanse, the universe shifts with every passersby, new isles are generated for every arrival, and in some cases - when a powerful enough individual enters them - they can stabilized and connected to the stable part of Elysium, expanding it. Some speculate that Elysium - to a degree - exists because it is observed, and without Simaar to observe everything at once, parts of this realm descended into the state of only theoretical existence.
There is no natural light source within Elysium, however there is also no darkness and no shadows - without any external light source, the natural state of Elysium on that field is eternal daylight (pleasant, with good visibility, but not light enough to blind you). It is not a normal light, either - it rejuvenates all forms of life, accelerates regeneration, fights back diseases, nourishes you (no need for food or water here), and weakens both curses and hostile magic.