In the generic D&D setting cosmology, as well as most specific campaign worlds, there exist several primal forces that compose reality, often called elements. These are the basic building blocks of reality. It’s assumed an even mix of elements leads to the creation of the Material Plane while imbalances create the various elemental planes.
Air, Earth, Fire, and Water (and para-elements, if you want to get technical) are fairly obvious. Less obvious are the alignment-based elements of Good, Evil, Law, and Chaos. Where most can can willingly accept things like fire elementals being composed of elemental Fire, most have more difficulty accepting beings of an elemental alignment, but they do exist after a fashion. It can be argued that an outsider with the Good subtype, such an an angel, or with the Evil subtype, such as a demon, are literally composed of that alignment element made flesh. After all, they count as either Good or Evil for all purposes regardless of their actual alignment.
In the Aldhaven campaign setting, these eight primal forces are recognized as distinct forces. It also adds two more: Light and Darkness. These two forces represent not only literal light and darkness but also positive and negative energy, which are not inherently good or evil in this setting, though many perceive them to be. The ten primal forces each have a related plane of existence, and all can produce elementals of their type, though the four classic physical elements remain the most commonly encountered.
Each element has is direct opposition, which should be fairly obvious. They are Air and Earth, Fire and Water, Good and Evil, Law and Chaos, and Light and Darkness. Where one is strengthened, its opposite is weakened, and when one is weakened, its opposite becomes stronger. All must remain in balance, to some extent, for the material realm to continue to exist and function as intended. They aren’t perfectly in balance, obviously, as there tends to be far more Water around than Fire, but they each have their roles to play in maintaining balance.
One last thing to note, the primal forces are not gods. They are not beings of any type. They simply exist. The primal force of elemental Fire is fire, and all fire is a representation of Fire, having some small fraction of Fire within it. A deity that possesses the Fire domain is able to direct and influence the primal force of Fire, but he serves as the caretaker of the force, not its creator or sole arbiter, and he does not have complete dominion or control over it.
That said, the primal forces have a will of their own, sort of. As mentioned, they are neither conscious nor sapient, thinking beings. However, they do have tendencies. For example, fire burns. If you give it more fuel, it burns more, which increases the presence and influence of Fire, at least until the fuel has been consumed. Each primal force almost instinctively seeks to increase itself, but each also has distinct limits.
The one exception here is Darkness. Time and again throughout history, Darkness has seemed to be almost sentient, as if it has goals and enacts plans to accomplish them while occasionally ignoring the conventionally understood limits that all primal forces have. This is, of course, patently ridiculous. Still, its presence is tenacious and insidious to the point that one could almost believe that it is coming for you.