While wandering about looking for sparrows this summer, having read some Moorcock shortly beforehand, I came up with the following brief sketch of a cosmology/pantheon based on the feywild/shadowfell idea:
The feywild and the shadowfell were once the only realities, themselves both sentient planes akin to overgods. Somehow, they collided with each other, creating a kind of big bang event that led to the prime material (for lack of a better name). The feywild is a place of chaos and eternal creation, in which very little is fixed, form is mutable, and change is the primary motivation of all matter and life. Before the collision, it was much more akin to a faerie version of the far realm, in which madness and chaos ruled and primal energies were not just important, but the only game in town. The shadowfell is a place of ending, decay, and eternal silence. Before the collision it was a plane of eternal emptiness and timelessness...infinite law.
When they collided, these entities changed each other, and created a plane the existed only as a subordinate reality, derivative of its parent planes. It was full of life, caused by the creative forces of the feywild, but these entities were mostly fixed in form and could die, thanks to the influence of the shadowfell. The feywild's chaos was somewhat mitigated by the infinite law of the shadowfell, and vice versa. Rather than being a plane of unlimited change, the feywild became merely chaotic. Instead of timeless emptiness, the shadowfell became an emptying, a place of wasting away into entropy, but infected with dark, shadowy forms of life like a mould infects an opened jar of jam.
The planes themselves, infinitely sentient, act like unapproachable gods. They do not answer prayers, and do not pursue agendas in the prime. They do however hate each other for corrupting their previous purity. Mortals honour the feywild as Nature, and she is worshipped by druids and the like. She is also seen as the source of all creativity and change, and is invoked when attempting these activities. They honour the shadowfell as Death, and also as Rust, the end of temporary things. While they ignore mortal prayers, they are formidable sources of power, which can be tapped into by adepts.
From the energies of the impact were also created a pantheon of gods, which I won't get into, but which represented a series of opposites. One god in particular, Asmodeus, was in charge of dying, in a grim reaper-like position. Knowing that his portfolio was powerful and unassailable, he challenged the shadowfell for control over death itself, and the plane punished him by trapping him in a prison deep within the plane itself. Minor beings that served him were turned into the devils, and imprisoned with him. Turning his eyes to the other gods, to prevent future treachery he linked their power to their portfolios.
The gods quickly learned that their existence was dependent on the presence of these opposites in the world, and many of them died or were destroyed by each other in the early days. Entire concepts, as powerful and meaningful as up and down, good and evil, hot and cold, were wiped from the prime, leaving only the opposites we are familiar with today. Some gods destroyed each other to steal their portfolios as insurance against annihilation, and others cultivated their existence by promoting their portfolios in sentient beings, concepts like love and hate.
Asmodeus has not abandoned his plans, however. He believes that the power of the feywild comes from her creation of souls, some of which come to inhabit bodies in the prime. The shadowfell is the destination of these souls, and he gains his power from their destruction. Asmodeus believes that by collecting enough souls he can eventually amass enough power to usurp the shadowfell's portfolio, thereby freeing himself and replacing the sentience of the plane he is trapped inside. Because of the lawful, but corrupted nature of the shadowfell, the devils are bound by rules, but imperfect rules with loopholes: devils can be freed by performing the appropriate rituals, but only for short periods of time. Asmodeus makes sure that mortals learn these rituals, and provides them with power in exchange for souls, using devils as intermediaries.
