Nobilis' cosmology is based on the Yggdrasil, the World Ash, an immense tree on whose branches all the worlds rest. Whether this is a literal or metaphorical construct is up to the GM to determine.
There are seven primary "factions" described in Nobilis: Heaven, Hell, the Light, the Dark, the True Gods, the Wild, and Aaron's Serpents. These represent the various affiliations of Imperators, the godlike beings who intervene in Earthly affairs.
While I find the World Ash to be marvelously poetic, I find it unconvincing as a model for the universe, and thus would probably say it is only a metaphor if I were to run a Nobilis game.
Furthermore, while I like Heaven and Hell and the True Gods well enough, I'm ambivalent about the Wild and Aaron's Serpents, who seem to lack clear origins and motives to me. I particularly dislike the Light and Dark factions, however. I'm also not satisfied with the role of Lord Entropy in Nobilis.
The reason I don't like Nobilis' Light and Dark sides is first of all, there seems to be insufficient contrast between Heaven and the Light, and between Hell and the Dark. Yes, they have slightly different motives and different ultimate goals, but in practice their behavior will often be very similar. Furthermore, the Light and the Dark are much too humanocentric for my tastes; why should almighty powers capable of shaking the universe be overly concerned with the fate of humanity in particular?
For another dualistic set of powers as an alternative to the conflict between Heaven and Hell, I'd prefer to use truly alien factions. Thus, I present the factions of Order and Chaos.
I also don't like Lord Entropy as described in Nobilis. He's supposed to be the big, bad leader of the Council, essentially the #1 Imperator on Earth, yet no reason is given why he gets to call the shots and create rules for everyone else. Is is just because he's more powerful than everyone else? Not much is given about his personality and motivations. He's just a deux ex machina for the GM to use to impose arbitrary limits on the setting.
To me, "entropy" suggests a primal force that should extend well beyond the domain of Earth. Hence, in my alternate cosmology, I would make Lord Entropy the chief "villain" of the setting, the entity who really wants to destroy all of Creation. The Excrucians might actually be his minions, or mere allies, or I might replace the Excrucians entirely.
This alternate cosmology is based partially on a Champions campaign I ran for over seven years, in which I created an entire pantheon of ultra-cosmic beings -- including, oddly enough, a character known as Lord Entropy.
In the beginning, there was nothing.
Then something came into being in the infinite void. Neither matter nor energy nor anything in-between, it was something, as opposed to the nothing that had previously been all that was. It was the raw stuff of creation. How it came to be is immaterial; it simply was.
Creation expanded, growing infinitely, but without dimension or form or substance.
A consciousness of sorts developed within the fabric of creation. Some say this consciousness represented the birth of God. However, the consciousness was not sentient in any sense we can comprehend. It had only two driving goals, which eventually caused it to fragment into discrete entities who ultimately worked at cross-purposes.
The raw stuff of creation was chaos. It had no pattern, no rhyme or reason to it, it simply existed. When creation acquired the spark of consciousness, the beings who would later become known as the Lords of Chaos came to be. These beings have only one goal: the continuation and expansion of creation. However, they desire only raw, unordered creation. Existence bound into form and substance, given meaning and order, is a subtraction from chaos. Everything comprehensible to living beings is order; the Lords of Chaos would see all of what we view as creation swept away and reabsorbed into raw, chaotic creation.
Part of creation's consciousness conceived of creating patterns. They began spinning universes out of chaos, and seek to maintain the existence of these creations. They are the Lords of Order, and they wage a constant struggle against the Lords of Chaos, who would see all of Order's work undone.
Neither a Lord of Order nor a Lord of Chaos, one fragment of creation's consciousness conceived of an entirely different end-goal; the return of all creation to oblivion. The end of everything. The Lords of Chaos sometimes view Lord Entropy as an ally, for he can destroy worlds and universes with a thought. Yet his ultimate ambition is the destruction of everything, including Chaos.
In this alternate cosmology, the Valde Bellum is much as described in Nobilis, but the ultimate enemy is Lord Entropy, who seeks to destroy everything that is. Neither the Lords of Order nor the Lords of Chaos are allies of mankind, though. All three of these powers exist on too high and abstract a level to interact with such infinitesimal beings as Powers. Even the Imperators of Order and Chaos are miniscule pawns created to operate within reality, much as macroscopic beings might create nanotechnology robots to send into a microscopic environment. No one can interact directly with a Lord of Order or a Lord of Chaos; they are beings who exist on an ultra-cosmic scale. Likewise, Lord Entropy cannot be fought directly; he can erase universes with a thought. The Lords of Order and Chaos and Lord Entropy struggle with one another directly, on levels as far above those of Imperators and Powers as their struggles are above the microbial struggles of single-celled creatures.
The Excrucians are minions of Lord Entropy -- or perhaps they are something else.