All wizards must choose a particular cosmic source of power that they tap to fuel their magic. (For priests, it’s always their deity; druids and some rangers tap Nature directly, and lose some flexibility in Blessing thereby.)
Your choice of a realm of power will influence aspects of divination and summoning magic, the spheres that connect to the higher worlds. It will also have special effects value: major rituals performed using Nature or Ecos have a tendency to cause nearby plants to bloom and birds to burst into song, while those using Unnature or Necros lead to withering plants and (nameless) dead birds dropping from the sky.
Nature is the transcendent realm of life. Ecos is the shadow of Nature in the spirit plane, and is more closely tied to the local ambient forces of life energy; it is affected much more by local conditions than is Nature. Only theurgists can tap Nature; some thaumaturgists make use of Ecos. Thaumaturgists who tap Ecos sacrifice some raw power for certain flexibility: elementalists may only be able to summon nature spirits rather than elementals, but they can shape living wood, and illusionists may not be able to summon shadow beings, but their illusions are much more lifelike than normal.
It is very difficult to cast magic that harms nature by calling upon Nature; if you need it to make a sacrifice to defend itself, it helps to call up some of the local spirits and make your case.
Interesting effects of channeling Nature and Ecos include making light that is effectively sunlight, using the Elemental spheres (all four at once) to reshape living or dead wood...
Unnature is the opposite of Nature, a deliberate violation of the Way of things. Theorists speculate on alternate islands or worlds where unlife is the primary principle. It is lethal to beings of the natural world, and is the animating principle of the undead. Necros is the spirit-plane manifestation of Unnature just as Ecos is the manifestation of Nature. Necromancers tap into the power of Necros. Disruption of the natural order fuels the power of Necros.
Just as the wheel of Nature is turned by the Sun, the wheel of Unnature can be turned by Deathlight, a radiant darkness that illuminates the world of Deathsight to fine details and nurtures undead plants. Some islands languish under the Ritual of the Black Sun, which creates a huge orb of darkness that hovers over the island, rendering vision by normal light very dim and causing deathburn, a sunburn-like ailment involving corpselike pallor and rot, to living beings who spend too much time under its light.
Interesting effects of channeling Unnature and Necros include making Deathlight, reshaping dead flesh, creating zombies and other undead through Summoning, commanding undead through Summoning, For more inspiration, check out the Death schtick at T3.
The four elements are forces of earth, air, fire, and water. Elementalists are prized for their ability to perform impressive feats of construction and destruction; they have access to a wide variety of Blast effects.
Primal chaos, a force of turbulent, ever-changing transformation. When damaging animate targets, it does warping damage, providing a number of short-term warping points equal to the caster’s Magic + the Outcome. It also eats away slowly at physical objects. See the Flux schtick at T3 for more inspiration, though some of those may require high-level Illusion or Summoning to work with; the Weird Surgery and Cauldron of Life powers under Healing are also a possibilities for those who channel Chaos, whether they use the Life power or the Elements. The power of Infusion can render ordinary alcoholic beverages specially empowered to guide people to chaotic activity.
Eternal order, a force of crystalline, mathematical purity. When damaging animate targets, it does warping damage, providing a number of short-term warping points equal to the caster’s Magic + the Outcome. It also slowly reduces physical objects to crystal or geometric forms. See the Law schtick at T3 for more inspiration.
Shadow is the realm of forms, the favorite source of power for illusionists. (Inspired by Plato’s cave, the shadow monsters in AD&D, and more.) An interesting and related phenomenon is the shadow roads, a layer of reality that is not the spirit plane and yet not the real world: distance and time there are a bit odd, and it is possible to swiftly travel great distances by walking the perilous shadow roads. It is rumored that some wizards of mythic power have actually been able to create shadow roads, for they are surely too dangerous to explore normally...
Channeling the power of various deities can provide a number of possible effects— the magic rules at T3 have a bunch of inspirational ideas, including their Light schtick.